506 



LECTURE XXI. 



responding with the oral and anal apertures in the tunic of the As- 

 cidian : in fig. 189, g is the inhalent, and g' the exhalent, siphon. 



Citherea chione. 



The labial processes are shown at h : the stomach at i. A remark- 

 a-ble elongated amber-coloured body, called the crystalline style, is 

 indicated at k : it is contained in a special cyst, surrounded by cir- 

 cular fibres, with its free extremity protruding into the stomach. 

 Its homologue exists in a rudimentary state " as a detached piece of 

 cartilage" in the stomach of the Oyster ; its office, as conjectured by 

 its discoverer, being "to assist in the trituration of the food."* In 

 the Pholas it has the form of a folded plate. To pound and mix the 

 alimentary molecules with the gastro-hepatic secretions seems more 

 obviously to be its function in the Bivalves, in which the style is 

 fully developed and projects into the stomach like a pestle in a 

 mortar.f 



The intestine {l\ after a few convolutions amongst the lobes of the 

 liver and genital gland, forms the rectum {m\ which perforates the 

 ventricle of the heart (w), and passes dorsad of the posterior adductor 

 to terminate by the anal opening, at the base of the siphon g' . The 



* CCCXI. p. 17. t X, torn. i. p. 137 ; preps. Nos. 404 a. 487, h. 



