THE LAMELLIBRANCHIATA. 33 



after it has made its haemal bend, curves down towards the 

 neural side of the body, and opens into the atrium on that side 

 of the body, and behind the nervous ganglion. 



The outer integument of the Ascidians secretes upon its 

 surface, not a calcareous shell, but a case or " test," which may 

 vary in consistence from jelly to hard leather or horn. And 

 it is not one of the least remarkable characteristics of the 

 group that this test is rendered solid, by impregnation with a 

 substance identical in all respects with that " cellulose " which 

 is the proximate principle of woody fibre, and forms the chief 

 part of the skeleton of plants. Before the discoveries of late 

 years had made us familiar with the production of vegetable 

 proximate principles by the metamorphosis of animal tissues, 

 this circumstance was justly regarded as one of the most 

 remarkable facts of comparative physiology. 



The last common and distinctive peculiarity of the Ascidians 

 which I have to mention, is one which acquires importance only 

 from its constancy. The middle of the haemal wall of the 

 pharynx, from near the oral to the cesophageal end, in all these 

 animals, is pushed out into a longitudinal fold, the bottom of 

 which projects into a blood sinus, and has a much thickened 

 epithelial lining. Viewed from one side, the bottom of the 

 fold consequently appears like a hollow rod, and has been 

 termed the "endostyle" (m, Fig. 12). The functions of this 

 structure are unknown, but it has been noticed in all genera of 

 Ascidians hitherto examined. 



XI. THE LAMELLIBRANCHIATA. 



This group, comprising those creatures which we know as 

 mussels, cockles, and scallops, and all the fabricators of what 

 are commonly known as bivalve shells (except the Brachiopoda), 

 presents an important advance in organization. In all these 

 animals, the body, as is exemplified by the diagram (Fig. 13) 

 of a freshwater mussel (Anodori), is included within a mantle or 

 " pallium," which is formed by a prolongation of the dorsal 

 integument, a structure in principle quite similar to that 

 which we met with in the Brachiopoda. But there is this 



D 



