The Polycopidae and Cytherellidae are curious marine families of a pelagic habit, with biramous second antennae well adapted for swimming, and very broad. The first maxilla in the Polycopidae is also employed in swimming, while the second is modified into a branchial organ; the maxillae of the Cytherellidae are more normal in structure, but both carry branchial lamellae. The posterior limbs are altogether absent in Polycopidae, and in the Cytherellidae are only represented by the copulatory organs of the male.

CHAPTER V
CRUSTACEA (CONTINUED): MALACOSTRACA: LEPTOSTRACA—PHYLLOCARIDA: EUMALACOSTRACA: SYNCARIDA—ANASPIDACEA: PERACARIDA—MYSIDACEA—CUMACEA—ISOPODA—AMPHIPODA: HOPLOCARIDA—STOMATOPODA

SUB-CLASS II.—MALACOSTRACA.

The Malacostraca are generally large Crustacea, and they are characterised by the presence of a definite and constant number of segments composing the body. In addition to the paired eyes we can distinguish two pairs of antennae, a mandibular segment, and two maxillary segments composing the head region proper; there then follow eight thoracic segments, the limbs belonging to the anterior thoracic segments being often turned forwards towards the mouth, and modified in structure to act as maxillipedes, while at any rate the last four are used in locomotion and are termed “pereiopods.”[[84]] The abdomen is composed of six segments, which typically carry as many pairs of biramous “pleopods,” and the body terminates in a telson. Not counting the paired eyes or the telson, there are present nineteen segments. The excretory organs in the adult open at the bases of the second antennae, and are known as “green glands,” but in the larva maxillary glands may be present homologous to those which persist in the adult Entomostraca. This is the typical arrangement, but sometimes the maxillary glands persist in adult Malacostraca, e.g. Nebalia, Anaspides, and some Isopods.

The hepato-pancreatic diverticula are directed posteriorly, and not anteriorly as in most Entomostraca, and the stomach is often furnished with chitinous teeth and ridges forming an elaborate gastric mill, especially in the larger Decapods.

SERIES 1. LEPTOSTRACA.
Division. Phyllocarida.

Fig. [76].—Nebalia geoffroyi, ♀, × 20. A.1, A.2, 1st and 2nd antennae; Ab.1, Ab.6, 1st and 6th abdominal appendages; A.G. antennary gland; C, half of caudal fork; E, eye; G, ventral ganglionic chain; H, heart; I, intestine; L, upper liver-diverticulum; M, adductor muscle of halves of carapace; MX, palp of 1st maxilla; O, ovary; R, rostrum. (After Claus.)

The small shrimp-like Crustacean Nebalia, which is found burrowing in the superficial layers of sand in the littoral and sometimes the deeper regions of most seas, has been regarded, ever since its anatomy was made out by Claus,[[85]] as a connecting link between Entomostraca and Malacostraca, and has been placed in a separate group Leptostraca.