THE AMPHINEURA 61 



SUB-ORDER 2. CHAETODERMOAIORPHA. 



Aplacophora without distinct longitudinal ventral (or pedal) 

 groove, with unpaired unisexual gonad, with differentiated liver, 

 and with posterior cloacal chamber provided with two bipectinate 

 gills. 



Anatomy. The mantle covers the whole surface of the body, 

 which is therefore cylindrical and vermiform in appearance. The 

 hinder half of the body is a little stouter than the anterior ; the 

 posterior extremity swollen and bell-shaped, forming the widely 

 cloacal chamber. The whole body has a uniform covering of short, 

 compressed, calcareous spicules implanted in the cuticula. 



The mouth is anterior, terminal, and crescentic, owing to the 

 presence of a rounded ventral shield. Chaetoderma radulifera alone 

 is provided with mandibles. The buccal cavity, whose anterior part 

 is partially protrusible, bears on its floor a very peculiar radula, 

 which may consist of (a) a single large tooth (Fig. 43, C), upon which 

 two small teeth are placed (C. nitidulum and C. production) ; (b) a 

 single large tooth, upon which 

 is a row of teeth (C. guttu- 

 rosum ; (c) no large tooth, 

 several rows of three teeth one 

 behind the other (C. raduli- FlG 42 



/era) ; (d) several distichous chmtodmna n itiMu m , ix,ve... The cephalic 



rOWS of tWO teeth each (C. enlargement is to the left, the cloacal or pallial 

 7 ,, -\ m t i chamber (containing the concealed pair i if ctenidia) 



CnOilengen). 1WO pairs Of Sail- to the right. (From Lankester, after Graff.) 



vary glands, similar to those 



in the Neomeniomorpha, open into the buccal cavity. The diges- 

 tive tract is quite straight, and narrows towards the middle of 

 its course to form the intestine. Just before it narrows it receives 

 the duct of a more or less extensive hepatic caecum, which extends 

 backwards on the ventral side of the intestine. The hepatic caecum, 

 large in most species, is feebly developed in C. challengeri. The 

 anus opens in the median line in the cloacal chamber (Fig. 43, B). 



The heart is posterior and dorsal, and lies nearly free in the 

 pericardial . cavity. It is traversed by the retractor muscles of the 

 gills. In its main features the circulatory system resembles that 

 of the Neomeniomorpha. The posterior extremity of the body is 

 hollowed to form a bell-shaped cloacal cavity, which has a con- 

 tractile aperture and contains a pair of large branchiae placed 

 symmetrically right and left of the anus. Eacli branchia bears a 

 double row of branchial plates, as is the case in the Polvplacophora 

 (Fig 43, B). 



The two renal ducts are more evidently true excretory organs 

 than in the Neomeniomorpha. They originate from the posterior 



