192 INVERTEBRATE MORPHOLOGY, 



usually a globular cavity, whose wall is formed by a layer of 

 ciliated cells containing fat-globules and various other par- 

 ticles, probably absorbed food-particles, these cells being 

 covered externally by a layer of connective tissue. Into the 

 stomach there opens from either side the duct of a gland (gl), 

 whose secretion is probably digestive in function and which, 

 may be termed a digestive gland from its resemblance to 

 similarly located glands in other invertebrates. The stomach 

 opens below into the shorter or longer intestine, whose walls 

 are lined by ciliated cells ; and this in turn communicates with 

 the terminal cloaca, which receives in some cases the termi- 

 nations of the excretory tubules and may be contractile. 

 The cloaca opens to the exterior, usually on the dorsal sur- 

 face, near the base of the foot, though in some forms which 

 live within a case the intestine bends forward upon itself, so 

 that the cloacal opening lies further forward. 



The nervous system consists of a relatively large ganglionio 

 mass (Br) lying on the dorsal side of the oesophagus, from 

 which nerves pass anteriorly to the trochal disk, and posteriorly 

 to supply a dorsal sensory papilla, the calcar (Sp). In addi- 

 tion to this, two pairs of posterior nerves have been described, 

 one of which passes to a sense-organ situated on each side of 

 the body in its posterior third, while the other pair runs back- 

 wards on each side of the middle line to near the posterior 

 end of the body, giving off branches to the musculature as 

 it goes. Among the sense-organs eyes (O) are very generally 

 present, varying in number from one to several, and situated in 

 the region of the supraoesophageal ganglion, with which they 

 are connected. They consist of patches of red, brown, or 

 black pigment with which sensory or retinal cells are asso- 

 ciated, and which are in some cases covered by a refracting 

 lens formed as a special cuticular thickening. Other sense- 

 organs to which a tactile function has been ascribed con- 

 sist in their simplest form of one or several cells bearing 

 stiff cilia. A pair of such organs is usually present, one on 

 each side immediately above the ganglion of the lateral 

 nerves, and anteriorly in the mid-dorsal line just behind the 

 trochal disk a third occurs, the calcar (Sp), which frequently 

 is situated upon the extremity of a tubular extensible process 



