HISTOLOGY OF THE ENDODERM. 125 



being directed towards the axis, and arc seen to be arranged in a single zone at some distance 

 from the centre. They are manifestly simple tubular lacuuic interposed among the cells of the 

 endoderm, and destitute of special walls. They are irregular in size, one of them especially being 

 usually considerably larger than the others ; and Agassiz, who was the first to call attention to this 

 difference of size, lays considerable stress upon it, for he regards it as constant, and considers 

 the large channel to represent that which alone constituted the cavity of the Tubularia in its 

 young state. I have made many sections of Tubularia stems, and have always found the tubes 

 irregular in size, and in most cases one of them considerably surpassing the others, as affirmed by 

 Agassiz. Wright, who first distinctly drew attention to the tubular lacuna, represents them as of 

 equal size ; but his figur* must be regarded as merely diagramatic. I am not, however, prepared 

 to assent to Agassiz's view of the origin of the large channel. I believe, on the other hand, that the 

 simple somatic cavity of the young Tubularia is represented in the adult by the common chamber 

 at the summit of the stem, into which the longitudinal channels all open. It is only in the free 

 stage of Tubularia that its cavity is simple ; and immediately after it has become fixed the hydranth 

 is carried upwards by the development of a stem whose cavity exhibits, at the moment it can be 

 detected, the compound character of the adult. 



That portion of the endoderm which occupies the axis of the stem consists of large cells of a 

 spherical form, or by mutual pressure more or less polygonal ; they are filled with a clear colour- 

 less or slightly granular fluid, while the more peripheral portion of the endoderm is composed of 

 small spherical cells containing abundance of minute vermilion-coloured granules ; it is in this 

 peripheral portion of the endoderm that the lacunaj are excavated. The walls of the lacunae 

 are clothed with very long and distinct vibratile cilia. 



A structure which in all essential points resembles that just described in Tubularia indivisa 

 occurs also in Corymorplia nutans, the endoderm of the stem being here, as in Tubularia, 

 excavated by longitudinal lacunar canals (PI. XIX, figs. 1, 6, 7). These canals inosculate here 

 and there with one another ; they are much more numerous than in Tubularia, but towards the 

 base of the stem they liecome by mutual coalescence less numerous than in the distal part ; they 

 liejust within the ectoderm, and open above into a common cavity in the basal portion of the 

 hydranth. Here the endoderm which forms the floor of the hydranth-cavity rises as a broad 

 conical projection so as to nearly fill the posterior half of the cavity. The axis of this projection is 

 perforated by a narrow canal by which the cavity of the hydranth is continued to the summit of 

 the stem. At this point the canal becomes wider, and receives the longitudinal channels of the 

 stem. The peripheral portion of the endoderm, or that in which the canals are excavated, is com- 

 posed of small spherical cells with reddish-brown granular contents, while the rest of the endoderm 

 ,is composed of loose cells with clear colourless contents, and forms a thick pith-like column 

 which occupies the whole axis of the stem. I have not succeeded in detecting cilia in the canals, 

 but, from the very distinct currents which may be witnessed in these canals in the living hydroid, 

 I have no doubt of the presence of cilia here as in Tubularia. 



The superficial position of the endodermal canals both in Tubularia and Corymorpha renders 

 them conspicuous without dissection, even to the naked eye, for they give to the stem the appear- 

 ance of being marked by longitudinal bands from one end to the other. 



In Anfennularia antennina the coenosarc in the main stems presents also a canaliculated 

 condition, one, however, which differs in some important points from that of Tubularia and Conj- 

 morj]ha ; for, while in the two latter the canals of the stem are mere lacunae excavated in a 



