MICHIGAN ACADEMY OF SCIENCE. 101 



EMBRYOLOGY. 



The ova have their origin in any part of the endoderm below the gonophores 

 and migrate upward between the ectoderm and endoderm, until they reach 

 the gonophores. Here they remain till they ripen and are laid (PI. 1, Fig. 2). 



The origin of the sperm cells is a little more complicated. At the outset 

 the ectoderm of the gonophore begins to divide into two layers, the inner one 

 of which stains deeply anc is destined to form the sexual cells. This layer, 

 in turn, divides into two, an outer thin layer, consisting of a single row of 

 cells, and an inner one, which rapidly separates into several rows of cells. 

 These stain deeply and are known as spermatoblasts. They become specialized 

 to form the mature spermatozoa. 



The ova are fertilized at the moment of ejection. The polar bodies are 

 rapidly given off, cleavage takes place and a ciliated planula is formed. This 

 becomes attached at one end, elongates, and tentacles are formed at the other. 

 At the basal end prolongations are given off to form the beginnings of the 

 tvibular network. These subdivide promiscuously, the intervening spaces 

 being gradually filled in by the extension of the coenosarc, and the secretion 

 of spines and spinules. 



CONCLUSIONS. 



1. Hydactinia polyclina, though almost invariably associated with the 

 Hermit-crab, is capable of an independent existence. 



2. In every Hydractinia colony there are normally present four kinds of 

 polyps : 



Gasterozooids. 

 Blastostyles. 

 Dactylozooids. 

 Tentaculozooids. 



3. The function of the gasterozooids is to collect, digest and absorb food 

 for the entire colony ; that of the blastostyles is reproduction. The function 

 of both the dactylozooids and tentaculozooids is to defend the colony against 

 enemies, the latter being especially adapted to this service by reason of its 

 great length. 



4. Judging from the accounts of the many investigations made upon 

 Hydractinia echinata, we have concluded that it is identical with Hydractinia 

 polyclina. 



