126 



MORPHOLOGY. 



Fig. 54. 



fomnion endodemi, the whole cfcnosarc is in Anfemmlaria hri)ken up into sepiu'ate tubes, cacli 

 with its own endoderm and ectoderm, and all surrounded by a common chitinous perisarc 

 (woodcut, fig. 54). 



Twenty or more tubes will thus be found in 

 the main stem of a specimen of Antenmdaria. 

 They lie close upon the inner surface of the peri- 

 sarc, and are connected to one another by an 

 extension of their ectoderm (woodcut, fig. 54 b). 

 In some parts of their course they are straight and 

 parallel ; in others, more or less sinuous and re- 

 ticulated by inosculation. The whole centre of 

 the stem is empty, and Las no communication with 

 the somatic cavity, instead of being occupied, as 

 in Tiihidaria and Corymorpha, by an endodermal 

 j)ith-like core, or instead of being, as in most of 

 the Hydkoida, occupied by the somatic cavity 

 itself. Very distinct currents may be witnessed 

 in these tubes, especially in the yoiuig hydroid, 

 thus affording evidence of the existence of vibra- 

 tile cilia on their walls. 



Endoderm of tentacles. — In the tentacles of 

 the Hydroida, both those of the hydranth and of 

 the medusa, the endoderm usually undergoes a 

 ])eculiar and important modification. In Hydra 

 the tentacles are quite pervious throughout their 

 entire length, while they are lined by a simple 

 continuation of the endoderm of the digestive 

 cavity ; but in every other hydroid with which 

 I am acquainted, the endoderm of the tentacles 

 of the hydranth presents a peculiar septate ap- 

 pearance, looking in some cases like a solid coi'e 

 divided by numerous transverse septa into cir- 

 cular discs, and entirely filling up the tube of the 

 tentacles, while in others, though the same septate condition exists, there is still a narrow pervious 

 canal in the axis. 



I believe that in all these cases the apparent septa are really the opposed walls of large 

 adjacent endodermal cells which encroach more or less on the cavity of the tentacle, often even to 

 its entire obliteration. 



This structure may be well seen in the tentacles of various species of Corynidce [Coryne 

 pusilla, PI. IV, fig. 3, &c.). For some distance from the distal extremity of the tentacle, 

 the cavity is usually entirely obliterated by thin membranous septa which stretch transversely 

 across it. Further towards the base of the tentacle, the septa become less regular, and the 

 obliteration of the tube is now plainly seen to be caused by the large irregular cells of which the 

 endoderm is here composed, and which encroach upon its cavity to such an extent as entirely to 



Canalliculatioii of Endoderm iu Antennularia antennina. 



A. Very young hydroid, just after the fixation of the planula 

 and the elevation of the primordial stem, a, a, the canals of 

 the endoderm, inosculating here and tliere with one another; 

 b, common cavity, formed by the coalescence of the endo- 

 dermal canals at tlie summit of the stem ; c, commencement of 

 a ramulus; d, lobed hydrorhizal disc, forming the surface of 

 attachment of the young hydroid ; e, circular groove, commence- 

 ment of segmentation in the stem. 



B. Part of a transverse section of the adult stem, a, ecto- 

 derm, and, i, endoderm of coenosarcal canals ; o', extension of 

 the ectoderm in the intervals of the canals ; c, perisarc. 



