HYDROIDS FROM WOOd's HOLL, MASS. 347 



be again referred to under that head. There is no special 

 differentiation at the end for attaching the animal. The coelen- 

 teric cavity extends to the tip of the foot end, and in it con- 

 stant circulation may be seen, due to the flagella on the endo- 

 derm cells. 



A very delicate perisarcal envelope covers the whole hydro- 

 caulus from the collar to the foot end, or it may even extend 

 farther beyond in a collapsed condition, adhering to foreign 

 objects, showing the distance the animal has travelled. It 

 invests the body so closely and is so thin that it can scarcely 

 be distinguished from the transparent ectoderm which secretes 

 it, except when favourable conditions of illumination show it 

 thrown into folds on the concave side of the body as the latter 

 bends in any direction. When polyps are roughly handled 

 with the pipette it is torn into shreds ; in sucli specimens it 

 first came to my notice. Of course indisputable evidence of 

 its presence and its tubular nature is found in the remains 

 left behind on which foreign matter has collected. Frequently 

 several of these may be found radiating from near the same 

 point, usually a mass of debris, which then marks the place 

 where several polyps from one parent leaving these tubes 

 originally stood (cf. fig. 10). 



The temporary nature of tbe perisarcal tube, which is easily 

 lost or even left, and quickly replaced, indicates that it simply 

 is a somewhat hardened mucous secretion serving for support 

 and protection, and not a true chitinous perisarc, such as other 

 hydroids usually possess. 



Sexual Reproduction. 



In Hypolytus the sexes are separate, and the males seem to 

 preponderate. Sexual reproduction probably takes place in the 

 latter part of summer, for by the middle of August sperm and 

 ova were just beginning to mature in some individuals. Only 

 one specimen was found with what appeared to be nearly 

 mature ova, and an attempt was made to fertilise them, but 

 was not successful (fig, la, g'"). 



The gonophores (fig. \,g',g") are limited to a narrow zone 



