﻿286 CONTRIBUTIONS TO THE NATURAL HISTORY 



diameter of the body is smallest ; the dotted circle outside indicating the greatest periph- 

 ery of the animal. In Fig. 8, the vertical tubes are intersected in the lower part of 

 their vertical branches ; but in Fig. 7, through both the ascending and the descending 

 branch, just above the main digestive cavity. In this way, the true relations between all 

 parts are easily appreciated. 



In Fig. 7 we see the circular muscular fibres (a) and all the vertical bundles cut 

 transversely, the external (6) as well as the internal (c), the main bundles as well as the 

 secondary ones. 



In Fig. 8, instead of the main digestive cavity, we have in the centre the lower par- 

 tition (a), the main nervous cord (6), the circular chymiferous tube (c), and the four 

 bulbs (a*), fully displayed. But the sections of the vertical muscular bundles (e, e) are 

 much smaller, and the lateral walls are much thinner. 



Fig. 9 and 10 correspond to Fig. 7 and 8, respectively ; showing the same parts under 

 the corresponding letters, with this difference, however, that, the upper arch of the chym- 

 iferous tube being uniform in Sarsia, the section which passes through the main digestive 

 cavity lays open, for a considerable distance, the inner cavity of the tubes themselves, as 

 well as that of the digestive sac ; if it had been made a little lower, the whole central 

 cavity would have been removed, and only one section across the vertical tubes would 

 have been shown, and not two, as in Hippocrene, the tubes forming no cross in their up- 

 per region. Again, in Fig. 10, when contrasted with Fig. 8, we see, in the centre, a trans- 

 verse section of the proboscis, as this hangs almost constantly lower down than the lower 

 omline of the main mass of the body; but otherwise, all parts are precisely identical. 



The alternate generation of Hippocrene has not yet been traced directly, so that I 



am unable to say whether this species originates from a Hydroid Polyp, similar to Cory- 



na?, or from one of the new genera described by Sars ; or whether the suggestion which I 



made in my Lectures on Embryology, that they arise from Tubularia, is a correct view 



of the case. From the greater number of tentacles in Hippocrene, from their more 



prominent connection with the margin of the bell-shaped disk, and from the numerous 



fringes around the opening of the mouth, I was led to suppose that the Tubularia? are 



really the polypidoms from which Hippocrenae are produced. I feel still more inclined to 



consider them so, as specimens of Hippocrene are scarce, when those of Tubularia are 



very abundant ; and for the reason that the reverse is the fact with Sarsia and Coryna, I 



should say that we are justified in considering them as sustaining that relation to each 



other. For those animals which pass through alternate generations in the course of their 



development will prosper either in one or the other state; and the one which is the most 



prominent feature in their existence brings, as a natural consequence, the less frequent 



