GENEEAL ANATOMY. 3 



hinder extremity, convex on the dorsal and flattened on 

 the ventral surface. The degree of dorsal convexity, 

 however, is variable, the most common form of the 

 body being sub-cylindrical, but in many cases — notably 

 amongst the Poecilostoma, Siphonostoma, and in some 

 genera of the Harpacticidge (Zaus, Peltidium, Por- 

 ceUidmm, Idya, ScuteUidium) — the animal is markedly 

 flattened, constituting a type of structure almost as 

 widely distinct from the normal Copepoda as are the 

 Isopoda from the Amphipoda, amongst the sessile-eyed 

 Crustacea. But in the genera of Harpacticidge here 

 referred to, the flattened form does not coincide with 

 any deep-seated differences of structure, for, indeed, in 

 some normal genera (Thalestris, Laoj)honte) we find 

 certain species assuming this form, but retaining all 

 the other characteristics of the genus to which they 

 belong. The directly opposite type of structure, in 

 which the animal is compressed laterally, occurs only 

 in the curious genus Amymone, a group of rare occur- 

 rence so far as our present knowledge extends, not 

 having been noticed outside of the European area. 



The front of the body is covered by a mem- 

 branous shield or carapace, which falls rather loosely 

 over the bases of the limbs below, but behind 

 is continuous with the coverings of the thoracic 

 somites; in front it is usually projected from the 

 forehead in the middle line, forming a rostrum, short 

 or entirely absent in some few cases, but often 

 moderately long and curved. In the Calanid^ it is 

 usually long and slender, much curved, and furcate 

 nearly to its base. Behind the rostrum is commonly 



