188 HYDROID^. Part IV. 



face of the walls. These granules frequently become loosened, and are borne along 

 in the circulation, and others keep up a constant quivering motion, as if disturbed 

 by the agitation of some neighboring body. The general rosy tint of the com- 

 munity is due to the presence of these brownish-red granules. The outward 

 opening of the chymiferous canal is above, and constitutes the mouth, serving also 

 for the exit of refuse matters. It is situated at the apex of a conical eminence, 

 which projects considerably beyond the region of the tentacles. The border of 

 the mouth, and the cone itself, is perfectly smooth (PI. XVII. Fi(fS. 9, m, 11, m, 

 and 11", m), and free from aj^pendages of any kind. 



The prehensile organs, or tentacles (PI. XVII. Figs. 2, i, 9, /, and 11, t), have 

 evidently a spiral arrangement, upon the head, but according to what order or 

 combination cannot be absolutely determined, on account of the protean shapes 

 which the head assumes. From all appearances, however, the i arrangement is 

 probably the order in which the tentacles are disposed. This agrees also with 

 the numbers in which the tentacles are developed ; first, two appear, then two 

 more, next, four more, making eight in all, and these last being duplicated, make 

 sixteen, the highest number usually observed. These being arranged upon the i 

 formula would account for the cross-like appearance that frequently prevails in their 

 disposition. The fact that the first two tentacles are developed apparently oppo- 

 site to each other (PI. XX. Fig. 4, t), seems to confirm this view.^ Although the 

 tentacles are developed in geometrical proportion, commencing with two, next four, 

 and then eight, &c., yet they are not, nor need they be, arranged on the head 

 symmetrically, in the order of their development, since the growth of their base 

 of attachment, may modify their apparent connection. Again, in all probability the 

 tentacles, besides being not exactly opposite in the beginning, do not originate, in 

 the first instance, simultaneously Avith each other. Owing to their great contractility 

 and the variable shape of the head, it has not been possible, so far, to determine 

 their exact relation to each other, as may be done with the rigid and fixed parts 

 of a plant. The axis of the tentacles is solid, and does not, therefore, admit the 

 circulation of the chymiferous fluid into their interior, as is the case with the 

 tentacles of the free Medusae budding from these Hydroids. The globular tips of 

 these organs serve, chiefly, to seize the prey, being filled with a multitude of lasso- 

 cells (PI. XIX. Figs. 2, /, 3, /, 5, and 5"), from which the long lasso-threads shoot out, 

 and coil around their victim, whilst the lower tapering part of the tentacle serves 

 to embrace and force into the mouth whatever may be caught. The contractility 

 and extensibility of the tentacles is remarkable ; at one time they stretch out as 



^ See Alexander Braiin's '■ Das IndiviJiiuni Akatleniie der Wissenschafleii zu Berlin vom Jalire, 



der Pflanze," in the " Abliandlun"; der KOniglichen 1853." 



