386 



Dr. E. JB. Wilson. 



[Dec. 14, 



which is absorbed by the peripheral cells by a process which appears 

 to be identical with the amoeboid absorption of yolk observed by . 

 Reichenbach in the embryo crayfish. At this period the gastric cavity 

 is without any communication with the exterior. 



II. 



(7.) The oesophagus is formed as a solid invagination of ectoderm, 

 in which a slit-like cavity, elongated in the dorso- ventral plane, soon 

 appears. The lower end of the oesophagus is then absorbed, placing 

 the gastric cavity for the first time in communication with the exterior. 

 Much variation exists in the process of absorption. Most commonly it 

 begins at one side, so that the bottom of the oesophagus hangs down 

 like a valve from the opposite side. It is then absorbed bodily. 



(8.) The radial septa and the horizontal or peduncular septum differ 

 widely in structure and mode of origin. The former arise simul- 

 taneously at the anterior end and grow backwards. Each septum 

 consists of two layers Of endoderm cells, separated by a structureless 

 lamella. The peduncular septum arises at the posterior end and grows 

 forwards. It is composed of three layers of endoderm cells, the middle 

 of which atrophies. This septum is probably to be regarded as formed 

 by the fusion of the dorsal pair of radial septa. The septa have a very 

 marked bilateral arrangement. 



(9.) The mesenterial filaments are formed as endodermic thicken- 

 ings of the edges of the septa. After the formation of the mouth they 

 become continuous with the invaginated ectoderm of the oesophagus. 

 The filaments are arranged in pairs of different lengths and structure. 

 The dorsal pair appears last and develops most slowly. 



(10.) The tentacles appear simultaneously and are at first destitute 

 of pinnas. They arise as hollow cascal outgrowths from the anterior 

 extremities of the radial chambers. 



(11.) The calyx- teeth are formed in a similar manner but in a 

 definite sequence. The ventro-lateral pair first appear and then the 

 median dorsal tooth. The remaining two pairs appear nearly at the 

 same time, but the medio-lateral pair usually precedes the dorso- 

 lateral. Occasionally, however, the reverse is true. 



(12.) The muscles are entirely endodermic, with the possible excep- 

 tion of those of the tentacles. They are developed from the bases of 

 epithelial cells, as " epithelio-muscular " cells or myoblasts. The 

 cell-body, in many cases at least, becomes reduced to a small granular 

 mass enclosing the nucleus and closely applied to the side of the fibre, 

 and the entire muscle-element lies below the epithelium. 



The muscles are arranged in two sets, longitudinal and circular, the 

 latter being outside the former. The circular muscles form a nearly 

 uniform sheet, but the longitudinal fibres are at first arranged in 

 • definite tracts which exhibit a striking bilateral symmetry. 



