AND PHYSIOLOGY OF TRICHODINA. - 123 



of the membrane. In dead or dying specimens, this membrane becomes folded or wrinkled 

 (fig. 16, I 1 ) transversely, and then these since (I 2 ) overlap each other and appear to fork 

 more or less, or seem to be linear processes, divergent from the curved ends of the 

 hooks (h)} 



The other or posterior set of since (I* to I 5 ) is much more readily detected than the anterior 

 one, and the strice are about three times as numerous. They are so closely set together 

 that it is a difficult matter to count them, although viewed with a one-eighth of an inch ob- 

 jective. They extend, like those of the anterior set, over the whole breadth of the mem- 

 brane, and, terminating abruptly at the peripheral margin (t 1 , / 4 ), give to the thickened 

 edge a milled appearance. This milling is, moreover, rendered conspicuous by an incras- 

 sated, scalloped border (I 1 ), in which the striae (I 2 ) of the front set terminate. 2 The striated 

 membrane is very flexible, and is frequently made to undulate, apparently, by the successive 

 impacts of the vibrating cilia. 



The apparently most important members of the adherent organ are the hooks (h). They 

 vary in number from twenty-two to twenty-four, and curve in a direction which is diamet- 

 rically opposite to the upward coil of the vibratory organ ; i. c., they are Iceotropic. They 

 are separate pieces, of an X-formed (fig. 18, h, h z ) shape ; the upright part of the X being 

 the hook (h) proper, and the horizontal limb (h 3 ) the base of it. These X's are arranged in 

 a circle with their horizontal limbs all pointing one way, — i. e., the same as the upright part, 

 or hook, — and nearly or quite touch each other, according to circumstances. A spur-like, 

 slender point Qi 2 ) projects from the horizontal part, in the opposite direction, and is about 

 half as long as the latter. Along this spur and the convex side of the hook a broad, lunate 

 crest (k) arises, and, nearly filling the interval between two succeeding hooks, projects 

 peripberally beyond the tips of the latter. This crest is excessively faint, and not recog- 

 nizable as a distinct body unless the striated membrane is removed ; although it is to be 

 seen when in place, especially where it projects beyond the tip of the hook, and forms with 

 the others a succession of scallops (fig. 17, k), lying in a circle parallel with the margin 

 (J}) of the striated membrane. 



Immediately within the row of hooks a series of nail-shaped pieces (i 1 , i 2 ) extends in a 

 circle ; and they are arranged in such order that each one lies opposite the horizontal part 

 (Ji 3 ) of a hook. The pointed, conical head (i 2 ) of the nail-shaped piece corresponds in 



1 In the Proceedings of the Boston Society of Natural which no one would for a moment think of separatinff Tricho- 



History for November 6, 1850, p. 354, Prof. L. Agassiz dina) must be removed to the class of Aealephse ! We must, 



makes the following statement in regard to the relation of for our own part, however, unequivocally dissent from this 



Trichodina to the Medusa; ; and especially in reference to view, since it is quite at variance with our own observations, 



these apparently forked, radiating stria;, which remind one of But again, according to another more recent statement of 



the numerous radiating tubes of certain Hydroid Acaleplue. Prof. Agassiz, in his Essay on Classification (Boston, 1857, 



He says, — " These parasites at times leave the Hydra, and p. 72; London ed., 1859, p. 108), he has satisfied himself of 



swim free, changing their form in a remarkable degree. In the "propriety of uniting the Vorticellida; with Bryozoa," i. e., 



addition to the internal ring, he was able to trace rays going the group of Vorticellidffl ; and consequently the Acalephan 



from the hooks to the margin, divided into numerous branches, (vide preceding paragraph) Trichodina — is Molluscan ! From 



and also rays proceeding toward the centre from this ring; the this view, also, we would modestly, but unequivocally dissent; 



margin has a fringed undulating edge, under the tentacles. By not only as the result of our own investigation, but in accord- 



feeding them with colors, he was able to see that the internal ance with the observations of other very competent authori- 



folds are the margin of a mouth, as in Rhacostoma, so that ties. This view would also seem to argue that the Bryozoa — 



these parasites on Hydra are diminutive Medusa;. In the egg if they do not strictly belong, with Polypi, to the division Ra- 



of Hydra, he had been able to trace all the forms from a seg- diata, as is insisted upon by other, and eminent authority — 



menled yolk to these parasites; the fresh-water Hydra is the are at least a transitional group between Radiata and Mollusca. 

 Polypoid form of Medusa;, while these parasites are the ~ The separation of these two sets of stria;, or radiating 



Medusoid form." ridges, is an excellent test of the quality of a quarter-inch 



If this be true, then the whole group of Vbrticellidse (from objective ; a one-eighth of an inch lens can do it easily. 



