TKUCHOPHOUK OF HYDROIDES UNOINATUS (EUI'OMATUS). 559 



Conn (9) pointed out that in Serpnla the egg-chorion is 

 never thrown off, but remains as the cuticle of the larva. 

 The gastrula has three noticeable featnres. The blastopore 

 is not round but slit-like, and arranged round its margin is a 

 circular band of locomotor cilia. Right opposite the blasto- 

 pore is the apical thickening, bearing a tnft of hair-like cilia. 

 The growth of the gastrula is not accompanied by elongation 

 in the main axis, but obliquely to this in such a way as to 

 pass through one end of the slit-like blastopore. One end of 

 the blastojiore is thus carried backwards away from the 

 other, which remains more or less fixed. The blastopore 

 becomes an elongated slit, the lips of which meet in the 

 middle and close, forming the rudiment of the future gut. 

 ¥ov a short time the digestive tract remains attached to the 

 ectoderm throughout the length of the blastopore, but after 

 a little it only retains this connection at either end. With 

 further growth the embryo is converted into the trochophore. 

 The digestive tract becomes hollow and acquires two openings 

 to the exterior at the two points of its previous connection 

 with the ectoderm. That near the ciliated band becomes the 

 mouth, while the other becomes the anus. 



" Just before the formation of the anus a number of ecto- 

 dermal cells near the region of the future anus become 

 separated from the rest of the digestive tract and form amass 

 of cells lying outside the alimentary canal in the body-cavity. 

 Tiiese cells form the mesoderm. Some of these cells increase 

 in size and form stellate mesenchyme cells, and finally a few 

 of them stretch across the body-cavity near the anus, forming 

 a membrane which separates a small portion of the bod}'- 

 cavity from the rest, forming the anal vesicle. Occasionally 

 another partition grows across it, separating it into two 

 smaller divisions." Certain other mesoderm cells form the 

 true mesoderm. " They multiply quite rapidly, and soon give 

 rise to the mesoderm bands. One of the eye-spots develops 

 much before the other" (p. 671). 



Von Drasche (10), in 1884, gave an account of the develop- 

 ment of Pomatoceros, but the early stages and the forma- 



