4 peof. m. m. haetog on the 



Segmentation op Body and Appendages, and Voluntary Muscular System. 



The body of Cyclops is somewhat pear-shaped, i. e. ovoid in front, narrowed, cylin- 

 drical behind (PI. II. figs. 1 & 2). It is obviously thus divided into a fore body or cephalo- 

 thorax and a hind body or abdomen, but the boundary is not easy to fix. Before a detailed 

 description, I will run over the chief points. The cephalon bears five pairs of appendages : 

 antennules, antennae, mandibles, maxillae, maxillipeds. Dorsally it forms one continuous 

 piece fused with the first thoracic tergum to form the carapace, but ventrally the first 

 thoracic segment is distinct. Then follow four free thoracic segments, growing smaller 

 and smaller, the last, indeed, being smallest anteriorly, so that the joint between the fore 

 and hind body is in front of it. All these segments of the cephalothorax are convexo- 

 plane, the dorsal convex side being prolonged into pleurae. The remaining five segments 

 are cylindroidal and lack pleurae, and the first is united with the second in the adult 

 female, so that these five have been taken collectively as the abdomen. But the first 

 has in both sexes a pair of appendages, reduced, it is true, to the condition of genital 

 valves, and should, therefore be ascrihed to the thorax, if we wish to be consistent in 

 our nomenclature of the Crustacea ; and I shall accordingly regard this as the last or 

 sixth thoracic segment. We have, then, a cephalon bearing five pairs of appendages, a 

 thorax of six segments, the first united dorsally with the cephalon, the last (in the 

 female) with the abdomen, and four abdominal segments, the last bearing the paired 

 f ureal processes, in all fifteen segments. 



The next subject before us is the full description of this body, its appendages, and 

 muscles ; for describing them together is a more intelligible plan than relegating the 

 muscles to another section, which would seem a plan as rational as, in describing a 

 steam-engine, to make separate articles for the wheels and the rods. In the note below 

 the terms used for denoting the various processes of the cuticle are defined*. 



Cephalothorax. — As stated above, this is composed of five distinct pieces dorsally, 

 seven ventrally ; it forms an ovoid convex above, flattened below, truncate in front, widest 

 about the posterior end of the cephalon proper, narrowed behind the end of the fourth 

 segment ; the fifth is almost triangular, with the base posterior, and the sixth squarish. 



* For brevity I use the following terms for outgrowths of the cuticle : — 



Seta — the finest, flexible, cuticular prominences into which no hypoderm can be seen to extend. These may be 

 inserted singly in small circular pits, in which case I term them " circumvallate seke." 



Teeth — coarse, simple, rigid prominences, usually pointed, containing or not processes of hypoderm. 



Spines — much elongated teeth, usually articulate at base, with a distinct core of hypoderm. 



Hooks — spines curved at end. 



Saws — flattened spines serrated on one or both edges. 



Hairs — flexible processes containing a core of hypoderm, and often punctate, probably owing to an incompletely 

 articulate structure. 



Plumes — hairs fringed with fine setoe ; " pinnate," when the sete are confined to two opposite sides. 



Lancets — pale flattened hairs, shaped like the blade of a knife ; inserted by a constricted, filiform, dark-bordered 

 stalk. 



