698 



ZOOLOGY 



SECT. 



sive cavity, lined with a ccelomic epithelium, and divided into three 

 completely separated parts the pericardium, the genital cavity, 

 and the general body-cavity. 



Vascular System. The vascular system of the Aplacophora 

 is very rudimentary. There is a heart enclosed in a pericardium 

 (Figs. 612 and 615, peri) and composed, when best developed, of 

 an auricle and a ventricle. 



In Chiton there is a well-developed heart (Fig. 613, Jit.) consisting 

 of a median ventricle and two lateral auricles. The pericardial 

 cavity in which it lies is a space of considerable extent in the 

 posterior region of the body, below the two last valves of the shell. 

 The Nervous System consists in the^ Aplacophoja (Fig. 614, 

 A,B,C) of four longitudinal nerve-cords twu^tetial and two pleural. 

 These are connected together by an cesophageal ring, thickened 

 dorsally into a single or double cerebral ganglion ; and in front of 

 this is a second, more slender stomatogastric nerve-ring with small 



ganglia. The 

 pedal cords 

 (v.v) may pre- 

 sent in front a 

 pair of gang- 

 lionic thicken- 

 ings connected 

 by a commis- 

 s u r e, and 

 further back 

 -pe there may be a 

 D series of e n - 



FIG. 614. Nervous system of Amphineura. A, Proneomenia ; largements 

 B, Neomcnia ; C, Chcetoderma ; D, Chiton, c, cerebral ganglia ; -, -, 



I, I, pleural cords ; pc. posterior commissure ; s, stomatogastric United. Dy COm- 



missures. The 

 pleural cords 



(I, I) are connected behind, above the rectum, by a commissure 

 (p, c) which usually bears a median enlargement. Sometimes a 

 union takes place posteriorly between the cords of the two pairs. 

 There are no eyes, or statocysts, or tentacles. The dorsal epidermal 

 papillae are perhaps sensory. Some have a sensory frontal lobe and 

 a sensory pit or elevation in the middle line of the dorsal surface 

 near the posterior end. 



In the Placpphora (Fig. 614, D) there is an oesophageal nerve- 

 ring consisting of a thicker dorsal cerebral portion not differentiated 

 into ganglia, and a thinner ventral buccal commissure. The cerebral 

 part sends off nerves to the labial palps, the lips, and the buccal 

 apparatus. Two pairs of longitudinal nerve-cords, pedal and 

 pleural, are given off posteriorly : the former, from which arise 

 nerves to the foot, are joined by numerous commissures passing 

 beneath the enteric canal ; the latter, which send off nerves chiefly 



X 



commissure or ring, with ganglia ; v, v, pedal cords. (From the 

 Cambridge Natural History, after Hubrecht.) 



