THE HYDROMEDUSAE 



nutritive zooids of the colony, may possess capitate tentacles 

 (Millepora), generally four, six, or twelve in number, or may 

 be entirely devoid of tentacles (Astylus). The endoderm cells 

 near their mouths are swollen and secretory. The dacfylozooids 

 are generally devoid of mouths, and either have (Millepora) or 

 lack (Stylaster) capitate tentacles ; their endoderm cells are not 

 enlarged. In some genera two kinds of dactylozooids are dis- 

 tinguishable by size and position (Spinipora). 



Both forms of hydroid have strong retractor muscles, and 



■ 1 \Wr'^ 



Fig. 43«. 



Skeletons of Allopora (upjier left hanil), Errina (lower left liatid), and 

 Stylaster (right hand). 



large complex nematocysts ; they are composed of the usual body 

 layers, and are connected at their bases with the tubular coenosarc 

 by radiating tubes. 



The structure of the (jonophores (Hickson, 38) varies considerably 

 in different genera, but is apparently in all cases referable to a 

 simplification of the medusoid type, such as has been sketched in 

 Anthomedusae (p. 20). The gonophore is not known to be ever 

 freed; it develops neither velum, tentacles, mouth, nor sense organs; 

 a manubrium is not invariably present. In Millepora Murrayi the 

 gonophore is formed at the apex of a dactylozooid (cf. Limnocodium, 

 Scyphistoma), in the other forms hitherto investigated it is formed 



