exudation, a long stalk or peduncle, whereby the animal is attached to some solid object. The peduncle is cylindrical, of varying length, flexible, and furnished with proper muscles. In some species it is naked, but in others it is furnished with calcareous scales. At its free extremity the peduncle bears the "capitulum," which corresponds to the shell of the Balanoids, and is composed of various calcareous plates, united together by a membrane, moved upon one another by appropriate muscles, and protecting in their interior the body of the animal with its appendages. The thorax and limbs resemble those of the Balanus; but "slender appendages, which from their position and connections are homologous with the branchia of the higher Crustacea, are attached to, or near to, the bases of a greater or less number of the thoracic feet, and extend in an opposite direction outside the visceral sac (Owen).

All the Balanide are hermaphrodite, and this is also the case with most of the Lepadida, but some extraordinary exceptions occur in this latter order. Thus, in some species of Scalpellum the individual forming the ordinary shell is female, and each female has two males lodged in transverse depressions within the shell. These males "are very singular bodies; they are sac-formed, with four bead-like rudimental valves at their upper ends; they have a conspicuous internal eye; they are absolutely destitute of a mouth, or stomach, or anus; the cirri are rudimental and furnished with straight spines, serving apparently to protect the entrance of the sac; the whole animal is attached like the ordinary Cirripede, first by the prehensile antennæ, and afterwards by the cementing substance. The whole animal may be said to consist of one great sperm-receptacle, charged with spermatozoa; as soon as these are discharged, the animal dies.'

"A far more singular fact remains to be told; Scalpellum vulgare is, like ordinary Cirripedes, hermaphrodite, but the male organs are somewhat less developed than is usual; and, as if in compensation, several short-lived males are almost invariably attached to the occludent margin of both scuta.

I have called these beings complemental males, to signify that they are complemental to an hermaphrodite, and that they do not pair like ordinary males with simple females" (Darwin).

DIVISIONS OF CIRRIPEDIA.-(AFTER DARWIN.)

ORDER I. THORACICA.

Carapace, either a capitulum on a pedicle, or an operculated shell with

a basis. Body, formed of six thoracic segments, generally furnished with six pairs of limbs; abdomen rudimentary, but often bearing caudal appen. dages.

Fam. 1. Balanida.

Sessile, without a peduncle; scuta and terga (forming the operculum) provided with depressor muscles; the rest of the valves immovably united together.

Fam. 2. Verrucida.

Sessile. Shell asymmetrical, with scuta and terga, which are mov. able, but not furnished with a depressor muscle.

Fam. 3. Lepadida.
Pedunculated.

and terga, when present, not furnished with a depressor muscle. Other valves, when present, not united into a single immovable

case.

ORDER II. ABDOMINALIA.

Carapace, flask-shaped; body formed of one cephalic, seven thoracic, and three abdominal segments, the latter bearing three pairs of cirri, but the thoracic segments being without limbs.

Genus. Cryptophialus.

ORDER III. APODA.

Carapace, reduced to two separate threads serving for attachment. Body consisting of one cephalic, seven thoracic, and three abdominal seg. ments, all destitute of cirri. Mouth suctorial.

Genus. Proteolepas.

CHAPTER XXXIII.

SUB-CLASS ENTOMOSTRACA.

SUB-CLASS III. ENTOMOSTRACA (Gnathopoda, Woodward).The term Entomostraca has been variously employed, and few authorities include exactly the same groups of the Crustacea under this name. By most the division is simply defined as including all those Crustacea in which the segments of the thorax and abdomen, taken together, are more or fewer than fourteen in number-the parasitic Epizoa and the Cirripedia being excluded. By Professor Rupert Jones the following definition of the Entomostraca has been given :

"Animal aquatic, covered with a shell or carapace, of a horny consistency, formed of one or more pieces, in some genera resembling a cuirass or buckler, and in others a bivalve shell, which completely or in great part envelops the body and limbs of the animal. In other genera the animal is invested

with a multivalve carapace, like jointed plate-armour; the branchiæ are attached either to the feet or to the organs of mastication; the limbs are jointed, and more or less setiferous. The animals, for the most part, undergo a regular moulting or change of shell, as they grow; in some cases this amounts to a species of transformation."

The Entomostraca are divided into two great divisions, or "legions," the Lophyropoda and the Branchiopoda, with which the order Merostomata may be conveniently considered.

DIVISION A. LOPHYROPODA.-The members of this division possess few branchiæ, and these are attached to the appendages of the mouth. The feet are few in number, and mainly subserve locomotion; the carapace is in the form either of a shield protecting the cephalothorax, or of a bivalve shell enclosing the entire body. The mouth is not suctorial, but is furnished with organs of mastication.

This division comprises the two orders Ostracoda and Copepoda.

ORDER I. OSTRACODA.-Small Crustaceans having the entire body enclosed in a shell or carapace, which is composed of two valves united along the back by a membrane. The branchia are attached to the posterior jaws, and there are only two or three pairs of feet, which subserve locomotion, but are not adapted for swimming. A distinct heart is sometimes present (Cypridina), but is more usually wanting (Cypris and Cythere).

Little is known of the development of the Ostracoda, but the young of Cypris are said to be "shell-bearing Nauplius forms" (Claus), possessing only the three anterior pairs of limbs, but protected by a bivalve shell. As in other Nauplii, the third pair of limbs, though now locomotive, are ultimately transformed into the mandibles. They pass through several stages, with complete moults, before arriving at sexual maturity. The young of Cythere, on the other hand, have at birth the two pairs of antennæ and two pairs of jaws, with three pairs of rudimentary abdominal limbs.

The order includes the Cyprides (fig. 130, a), which are of almost universal occurrence in fresh water. The common Cypris is completely protected from its enemies by a bivalve. carapace, which it can open and shut at will, and out of which it can protrude its feet. The closure of the carapace is effected by means of an adductor muscle. Locomotion is mainly effected by means of a pair of caudal appendages. The Cypris is extremely prolific, and a single impregnation appears to last the female for its entire lifetime. Young females, produced in this way, are also capable for some generations of producing fresh individuals without the influence of a male (parthenogenesis).

ORDER II. COPEPODA.-Small Crustaceans, having the head and thorax covered by a carapace, and furnished with five pairs of natatory feet. Usually there are two caudal locomotive appendages. A distinct heart is sometimes absent (as in the Cyclopida), but is sometimes present. Both marine and fresh-water Copepods are known.

The larvae of the Copepods are Naupliiform, with unpaired eyes, three pairs of limbs (the future antennæ and mandibles), and two terminal setæ. Next the maxillæ are produced, and then three other pairs of limbs (the foot-jaws and the two front pairs of natatory feet). At the next moult, the larva assumes the Cyclops form, but has at first much fewer limbs and somites. In the Cyclops (fig. 130, c), which is one of the commonest

of the "Water-fleas," the cephalothorax is protected superiorly by a carapace, and the abdominal somites are conspicuous. In front of the head is situated a single large eye, behind which are the great antennæ and the antennules. The feet are five pairs in number, each consisting of a protopodite and a segmented exopodite and endopodite, usually furnished with hairs, and forming an efficient swimming apparatus. The young pass through a metamorphosis, and are not capable of reproducing the species until after the third moult or change of skin. The female Cyclops carries externally two ovisacs, in which the ova remain till they are hatched. A single congress with the male is apparently sufficient to fertilise the female for life.

The Copepoda, or Oar-footed Crustaceans, are all of small size, and are of common occurrence in fresh water in all parts of Europe. Many forms also live in the sea, sometimes in

immense numbers. Thus Cetochilus is so abundant in the North and South Atlantic, as to communicate a ruddy tinge to the ocean, and to serve as one of the principal articles of diet of the whale. By good authorities the Ichthyophthira are regarded as merely Copepoda peculiarly modified to suit a life of parasitism.

DIVISION B. BRANCHIOPODA.-The Crustaceans included in this division have many branchiæ, and these are attached to the legs, which are often numerous, and are formed for swimming. In other cases the legs themselves are flattened out so as to form branchia. The body is either naked, or is protected by a carapace, which may enclose either the entire body, or the head and thorax only. The mouth is provided with organs of mastication.

The Branchiopoda comprise the Cladocera, the Phyllopoda, and probably the Trilobita, though this order departs in many respects from the above definition. The Merostomata may be considered along with these, though these, too, are in many respects peculiar.

ORDER I. CLADOCERA.-The members of this order are small Crustaceans, which have a distinct head, and have the whole of the remainder of the body enclosed within a bivalve carapace, similar to that of the Ostracoda. The feet are few in number (usually four, five, or six pairs), and are mostly respiratory, carrying the branchia. Two pairs of antennæ are present, the larger pair being of large size, branched, and acting as natatory organs. The Cladocera quit the egg with the full number of limbs proper to the adult.

In the Daphnia pulex (fig. 130, b), or "branched-horned Water-flea," which occurs commonly in our ponds, the body is enclosed in a bivalve shell, which is not furnished with a hinge posteriorly, and which opens anteriorly for the protrusion of the feet. The head is distinct, not enclosed in the carapace, and carrying a single eye. The mouth is situated on the under surface of the head, and is provided with two mandibles and a pair of maxillæ. The gills are in the form of plates, attached to the five pairs of thoracic legs. The males are very few in number, compared with the females, and a single congress is all that is required to fertilise the female for life. Not only is this the case, but the young females produced from the original fecundated female are able to bring forth young without having access to a male. Two kinds of eggs occur in Daphnia. In the first of these, or "summer eggs," the ova (from ten to fifty in number) are deposited in an open space between the valves, and are retained there until