APLYSIA. 227 



The kidney is not the only organ of excretion. The 

 sacculations forming the crista aortae, certain cells of the liver 

 and of the connective tissue, and the opaline gland are probably 

 all concerned with the elimination of waste matter. 



The Crista Aortae (Fig. 13, cr. ao.) lies on the aorta close 

 to its origin from the ventricle of the heart. Leucocytes occur 

 in its walls and are thence transferred to the blood stream, so 

 that the region is also a lymph gland. It is claimed by Grobben, 

 however, that waste matter from the blood is drawn off here, 

 and that the crista aortae is homologous with the pericardial 

 gland of other Molluscs. 



The liver contains two kinds of cells : — (a) Protoplasmic 

 cells rich in granules, secreting a digestive ferment, and 

 (b) cells poor in protoplasm, but usually full of brown oily 

 globules, excretory in function. The former stain deeply, the 

 latter hardly at all. 



The connective tissue surrounds the whole of the body 

 organs as a thin layer and permeates between the coils of 

 convoluted organs, e.g., accessory genital mass, forming a 

 packing for the coils. It also fills the spaces between the 

 muscle fibres of the foot and parapodia, but not completely, 

 for frequent gaps, the lacunae, occur. These lacunae are 

 part of the blood vascular system. In the large haemocoele 

 (ventral abdominal sinus), in addition to the layer covering 

 the gut, nervous system, etc., layers of connective tissue 

 occur between the gut and body wall. (These have been 

 omitted from Figs. 21-23.) In many parts, especially where 

 the organs are compact, as in the visceral hump, the connective 

 tissue is fenestrated to allow of the passage of the blood through 

 it. The tissue consists of colourless corpuscles embedded in 

 a semi-gelatinous matrix. Some of these corpuscles (cells of 

 Leydig) contain concretions of chalky matter, and function as 

 accumulative excretorv cells.* 



* Note that in a specimen preserved in acid fixative all traces of these 

 concretions disappear. 



