310 A. T. MASTERMAN. 



and appear to be portions of the coelom, with the trunk 

 nephridial tubes cut off from the rest of the trunk coelom. 



These two organs are seen in a longitudinal section in 

 PI. 21, fig. 45. A study of other stages can alone show their 

 true significance. 



Skeletal System. — The skeletal elements are difficult of 

 demonstration, but a very thin layer of mesoblastic chondroid 

 tissue, homologous with that of the adult, appears to be pre- 

 sent immediately under the epiblast. In carefully macerated 

 specimens the epiblast of the tentacles can be removed, and 

 the skeletal system is then evident as a thin hyaline layer. In 

 the trunk region, again, it is difficult to account for the rigidity 

 of the body-wall, considering its extremely attenuated epiblastic 

 and thin mesoblastic layers, without the assumption of the 

 skeletal supporting layer. 



Muscular System. — The muscles are all in the condition 

 of simple mesoblastic cells with long contractile processes. 

 They are most prominent in the interior of the hood, where 

 they form a fine raeshwork. The thickest and most regularly 

 arranged are inserted in the mesentery, between the pre-oral- 

 and collar-cavities, and radiate outwards on either side to the 

 edge of the hood (figs. 23 and 50). A i'cw strands also connect 

 the further side of this mesentery with the sheath of the 

 notochord. In the collar there arc two muscular bands, 

 already referred to, attaching the collar nephridia to the collar 

 Mall, and there are a few muscle-cells forming a sheath around 

 the ventral blood-vessel (PI. 19, fig. 17). In the trunk region a 

 few fine circular muscle filaments can be discerned in surface 

 view, and a dorsal longitudinal trunk (fig. 16). The trunk 

 mesoblast, forming a sheath round the " diverticulum^^ ("foot,'^ 

 Caldwell), is very thick, and mesoblast cells are becoming 

 differentiated into true muscular elements (PI. 20, fig. 19, div.). 



The splanchnic mesoblast over the dorsal blood-vessel is 

 thickened by an accumulation of simple contractile cells (figs. 

 16, 17, and 20, d. I. v.). The contractile nature of the dorsal 

 blood-vessel has been noticed by many workers. Thus 

 Schneider found it to be rhythmically contractile, and remarked 



