ON THE STRTJCTURE OF HYDEACTINIA EOHINATA. 79 



paper on this hydroid that had so far appeared. He mentions 

 two polyps, — reproductive polyps without a mouth, and nutri- 

 tive polyps with a mouth ; but describes the chitin as an endo- 

 skeleton developing in the substance of the common coenosarc. 

 He says that in the nutritive polyps the tentacles vary in 

 number^ are distributed in two alternating circles, and are 

 very contractile. The polyps are all connected with a common 

 living tissue which is directly continuous witli the exterior 

 layers of the body, and which, towards the edge, only consists 

 of a delicate pellicle enclosing no solid skeleton. Below the 

 horny base, and protected by it, there are little anastomosing 

 tubes, the central canals of which communicate with the 

 digestive cavity of the polyps. 



De Quatrefages is the first observer who describes the 

 reproductive polyps in any detail. 



Van Beneden (8), in 1844, published a second paper, in 

 which he describes the two sexes of Hydractinia as two 

 distinct species — H. rosea and H. lactea. 



Johnston (9), in 1847, revived the specific name "echi- 

 nata,^^ used by Fleming. He did not accept De Quatrefages' 

 view of the endoskeletal nature of the chitin ; on the other 

 hand, he conveys no distinct impression as_ to how the indi- 

 viduals are connected together. 



Strethill-Wright (10), in 1856, published a paper on 

 Hydractinia, containing the most correct account of the 

 structure of the colony which had so far appeared. He 

 describes the corallum (skeleton) as being secreted externally, 

 and forming a raised network between the spines. The poly- 

 pary (soft tissue) '' invests the corallum and fills up the grooves 

 " of its papillse, the interstices between its reticulations, and 

 '^the cavities of its hollow spines. It is often absent at the 

 " summits of the papillae. It secretes, renews, and extends the 

 " corallum, gives rise to new polyps, and is the seat of commu- 

 '^nication between the polyps of the colony. ... I conclude 

 "that the polypary of this zoophyte consists of a single layer 

 '^ of endoderm enclosed between two layers of ectoderm. That 

 " the lower ectodermic layer, as it grows over the shell, attaches 



