THE ENTEROPNEUSTA. 17 



mesoblast cells which are in contact with the hypoblast into 

 curious tails, which are refractive. 



I was at first led to suppose that these tails were mus- 

 cular, and this undoubtedly is the fate of many of the 

 mesoblastic elements of this region, but no direct evidence of 

 the contractile nature of these pear-shaped cells was attained 

 beyond the general suggestion of their shape ; on the other 

 hand, when the notochordal sheath is developed in this region^ 

 an appearance is presented which suggests the possibility of 

 their having taken part in its formation. Their histological 

 characters are, however, strikingly similar to those of the cells 

 which occur at the sides of the notochord in the same region 

 in Amphioxus at the time when it has eleven pairs of meso- 

 blastic pouches (Hatschek, No. 4, figs. 124, 126, &c.). These 

 cells are stated by Hatschek to be muscle-fibres ; by analogy 

 it seems, therefore, likely that this may be the real nature of 

 the same cells in Balanoglossus. 



As development proceeds, proliferations of mesoblast are 

 formed in the ventral region of the middle body cavities. From 

 each side in the posterior region of these cavities a tubular 

 portion is separated ofi" from the rest (fig. 28). 



The fate of these parts is not quite certain. It seems, how- 

 ever, likely that they unite with two forward growths from the 

 posterior body cavities to form the tissue space in which the 

 dorsal blood-vessel is ultimately enclosed throughout the collar 

 region. This tissue space I propose to call the perihaemal 

 cavity (fig. 60, &c., Ph. c). 



The posterior body cavities are simple cavities, similar 

 to the middle pair. The dorsal and ventral mesenteries 

 persist throughout life. As yet they contain no special 

 differentiations. 



Period between the Formation of the First and 

 Second Pair of Gill-Slits. 



Skin and Nervous System. — The histology of the skin 

 in the proboscis and collar regions has not undergone material 



2 



