688 



ZOOLOGY 



SECT. 



mantle. The anterior end of the style, which projects into the 

 stomach, appears to be slowly dissolved by the digestive juice, 

 forming a sort of cement to enclose the hard particles of the food 

 and prevent any harmful effect on the mucous membrane. It is 

 possible also that the dissolved substance of the style may play 

 the part of a digestive secretion, since it appears to contain a 



substance of the nature of a digestive 

 ferment capable of acting upon starchy 

 matters. 



The excretory organs or kidneys 

 occur in their simplest form in the Proto- 

 branchia, in which they have the form of 

 cylindrical curved tubes, opening at one 

 end into the pericardium and at the other 

 on to the exterior ; the whole organ is 

 lined with glandular epithelium, and has 

 no communication with its fellow of the 

 opposite side. In the higher forms the 

 organ becomes differentiated into a secreting 

 portion or kidney, which is very spongy in 

 texture, and opens into the pericardium, 

 and a non-secretory portion or bladder, 

 which opens externally. Frequently there 

 is a communication between the right and 

 left nephridia, and in some genera, such as 

 the Oyster, the organs become extensively 

 branched. Also taking a share in the pro- 

 cess of excretion are the pericardial glands, 

 or Keber's organs, glandular developments 

 of the wall of the pericardium. 



FIG. 601. Nervous system and rSr-^Tilatrvr'v Or>crarG TliP linnrf is 



auditory " organs of NU- Circulatory urgans.- 



cuia. i, pieurai ganglion ; usually perforated by the rectum, but lies 



II, pleuropedal connective; , ,1 -vr i /TV i-^/-> T\ 



in, common connective from altogether above it in JNucula (-big. 595, //) 



ganglia V an iV^ leU "aumtory a> ' and some other genera ; the ordinary 



vi rv viscerai P ganiiou"- gl vii ' arrangement seems to have been brought 



posterior pallia! nerve;' vii i ', about by the heart becoming folded over 



osphradial ganglion; IX, ,-, . ' . -, -,111 , T J.T, 



visceral connective; x, stato- the intestine and united below it. In the 

 sta K-rtr S e ! Oyster and some other forms the heart is 



VI 



In Area the ventricle 



, its external apei 



xiii, cerebro-pedai connec- below the rectum. 



tive ; XIV, anterior pallial .,.._,. . 



nerve ; XV, nerve to palps ; is divided into two by a Constriction. 



XVI, cerebral ganglion. T, ?. r T ,1 t f 



(From Peiseneer.) rores are often lound on the surface ot 



the foot, and it lias been asserted that 



through them the external water mixes with the blood ; this, how- 

 ever, is certainly not the case : the blood-system is everywhere 

 closed. The blood is red in some forms (e.g., Area) owing to 

 haemoglobin in the corpuscles ; in some cases it is of a bluish 

 tint owing to the presence of haemocyanin. 



