Devehjyment of Mp-iotliela phiygia. 319 



8. The structure of the tentacles is in the highest degree 

 interesting. In their narrow stalk-like portion the condition 

 of the endoderm departs widely from that of this tissue in the 

 tentacles of other marine hydroids ; for it presents no trace of 

 the septate disposition so well marked in these. It is, on the 

 contrary, composed of a layer of small cells loaded with opaque 

 granules and surrounding a continuous wide, axile cavity. 



9. It is, however, in the terminal capitulum of the tentacle 

 that the structure of these organs departs most widely from 

 any thing that has as yet been recognized in the tentacles 

 of other hydroids. Here a very peculiar tissue is developed 

 between the muscular layer and the proper ectoderm, where 

 it takes the place of the zone of caudate cells. It forms a 

 thick hemispherical cap over the muscular lamella and en- 

 doderm of the tentacle, and is composed of closely applied 

 exceedingly slender prisms, with their inner ends resting on 

 the muscular lamella, to which the prisms are pei-pendicular, 

 the whole structure forcibly suggesting the rod-like tissue 

 associated with special sense-apparatus in higher animals. It 

 appears to be but a modification of the tissue which elsewhere 

 forms the zone of caudate cells. 



10. Extending in a radiating direction from the convex 

 surface of this rod-like tissue, towards the external sm-face of 

 the tentacle, may be seen numerous clear cylindrical rods, each 

 of which, making its way among the cells of the ectoderm, 

 terminates distally in a very delicate transparent oviform sac, 

 which carries near its distal end a minute styliform process. 

 Within this sac, and completely filling it, is an oviform cap- 

 sule with firm transparent walls, and having immersed in its 

 very refringent contents a cylindrical cord wound upon itself 

 in two or three coils. Under pressure the contained cord 

 may be sometimes forced out through the smaller or distal end 

 of the capsule. Notwithstanding the obvious resemblance of 

 these bodies to thread-cells, their significance is, without doubt, 

 something entirely difierent. Their assemblage constitutes a 

 zone parallel to the spherical surface of the capitulum, and 

 lying at a slight distance within it. Though it is impossible 

 with certainty to assign to them their exact function, we feel 

 compelled to regard the whole system, including the rod-like 

 tissue to which their stalks can be traced (and which is only 

 a modified portion of the nervous zone), as an apparatus of 

 sense. This is the only known instance of the existence in a 

 hydroid trophosome of any thing which may with reason be 

 regarded as a special apparatus of sense. 



11. The male and female sporosacs are borne by the same 

 trophosome. 



23* 



