38 LAMELLIBRANCHIA. 



growth ;md division of the large entoderm cells, which until now 

 have remained only slightly differentiated. These cells are, in Teredo, 

 retained in this primitive condition for a very long time (Fig. 15 B) : 

 it is evident that they contain, stored up in them, a rich supply of 

 nutritive material, which is gradually used up in the formation of the 

 larval body ; the presence of this food renders an early development 

 of the intestine, such as takes place in Oxfrea, unnecessary (Fig. 

 14). At first the intestine makes but a simple bend, and seems to 

 resemble in shape that of the Annelidan Troclwphore, but soon, as a 

 consequence of its elongation, it forms several coils (Fig. 18). 



We have, so far, left out of consideration the primitive inesoderm- 

 rudiment and its derivatives, which are, nevertheless, of great import- 

 ance. According to Rabl and Hatschek, the symmetrically arranged 

 mesoderm-bands run forward from the two primitive mesoderm-cells 

 [mesodermal teloblasts] which at first lie near the blastopore and 

 afterwards (vent rally) at the sides of the anus. The constituents of 

 these mesoderm-bands are, as in the Annelida, yielded by the division 

 of the primitive mesoderm-cells, which long retain unchanged the 

 character of the blastomeres (Fig. 15). The mesoderm-bands of the 

 Lamellibranchia do not appear so distinct or so highly developed as 

 those of the Annelida, since, from an early period, cells bud off 

 from the main mass of the mesoderm which become distributed in the 

 primary body-cavity.* These give rise to the muscles of the larva; 

 the originally round cells lengthen, send out processes, and, finally, 

 by assuming a fibrous structure, produce the fibres of the retractor 

 muscles (Figs. 15 C, and 18). The retractors of the velum which 

 run from the posterior part of the shell to the cephalic area form first. 

 Then several shorter muscles are added, also running from the inner 

 surface of the shell in the region of the hinge, and finding points of 

 insertion in the post-oral region of the body (Fig. 18). These muscles 

 seem to serve chiefly for closing the shell (Hatschek), but this func- 

 tion is carried out principally by the muscles which, soon forming 

 from long mesoderm-cells, traverse the body -cavity dorsally to the 

 intestine, running from one shell-valve to the other. This shell- 

 adductor appears very early in the larvae of many Lamellibranchs 

 (Fig. 16, sm). 



* [According to Lillie (No. III.), this larval mesoderm has in Unio an 

 origin quite distinct from the mesodermal teloblasts which form the mesoderm- 

 bands. The larval mesoderm has more the character of a mesenchyme, and 

 is situated in front of the blastopore, whereas the mesodermal teloblasts are 

 situated behind the blastopore ; the former gives origin to the larval muscles. 

 The position of the larval aud adult mesoderm is well seen in Figs. 22 and 

 2H .1.— Ed.] 



