Natural History of the Vorticellse. 199 



Zoothamniumj and especially to the possible relation of the 

 lump-like animals to reproduction. 



We have already indicated that, with the exception of 

 Astylozoon and Gerda^ all the Vorticellae present only attached 

 representatives. This attachment, and with it a limitation of 

 locomotion, is certainly their ordinary mode of life, which 

 governs the essential systematic character. But probably all 

 the Vorticellffi, and certainly most of them, pass temporarily 

 into a free life-stage, by forming the so-called hinder circlet of 

 cilia, separating themselves from their peduncle, and swimming 

 about freely in the water for a time, until they again attach 

 themselves by secreting a peduncle, and at the same time very 

 quickly lose the posterior circlet of cilia, the essential attribute 

 of their freedom. This separation takes place either on the 

 occasion of their division, as is always the case in Vorticella^ 

 as one of the divisional buds must quit the peduncle which is 

 only intended for one individual, or particular individuals of a 

 stock, usually when injured and disquieted in their ordinary 

 conditions of life, spontaneously quit the colony to seek their 

 fate elsewhere. 



External Covering and Musculature. 



All the Vorticellse possess an external, hyaline and homo- 

 geneous skin, which is pretty strong in many species, which 

 covers the whole body, passes posteriorly into the sheath of the 

 peduncle when the latter is present, and affords the axis of the 

 peduncle access to the base of the body, and anteriorly wraps 

 round the peristome, clothes the ciliated disk, and is continued 

 into and lines the nutritive canal. This skin may be brought 

 into view in the living animals by suitable magnifying-power, 

 but also, and generally still more distinctly, by means of various 

 reagents (acetic acid, solution of potash, &c,), to which it offers 

 considerable resistance. By the addition of colouring materials 

 or iodine, also, it becomes very distinct, as it remains untouched 

 by them and contrasts with the contents, which are rapidly and 

 intensely colom'ed, and it then appears as a colourless hyaline 

 border at the limits of the body. 



Probably in all Vorticellse this skin exhibits a regular 

 transverse striation running round the whole body ; and this is 

 often so fine and close that it is detected only by a high power 

 and close examination ; sometimes, however, it shows stronger 

 outlines. This striation has already been seen and described 

 by Ehrenberg, and after him by many others ; and with a little 

 experience it cannot be confused with other coarser folds, also 

 appearing in transverse rings, which are chiefly produced by 

 sudden contraction after previous compression ; for this constant, 

 regular, and fine striation of the skin may be seen when the 



