PROTOZOA. 59 



It is interesting to note that in one species of this 

 genus, Vorticella umbellaria C. & L., there are nemato- 

 cysts or thread cells which are more effective weapons 

 than the tricocysts. Each of these cells contains a 

 spirally wound thread, like the thread cells of the more 

 specialized Coelentera soon to be described. 



The compound colonial form, Zoothamnium alternans 

 C. & L. (PI. 57), is, according to Kent, one of the most 

 remarkable instances of polymorphism among the Infuso- 

 ria. In this genus there are three differentiated forms of 

 zoons. PI. 57 shows two of these forms. The large 

 size of the macrogonidia in this species is unusual. 



INFUSORIA. TENTACULIFERA. 



The Tentaculifera are represented in the Collection by 

 the Podophrya gemmipara Hertwig (PL 58, figs. 1-4, 

 a-e). By the possession of cilia, the young form (fig. i) 

 shows its probable relationship with the ciliate Infusoria. 

 In the course of development the cilia disappear. Kent 

 observed, however, in specimens obtained from North 

 Wales in 1 88 1, that the embryos were provided with 

 short tentacles either in addition to or in place of a more 

 or less conspicuously developed ciliary covering. Fig. 2 

 is a young form showing the origin of the stem. The 

 adult (fig. 3) is much more differentiated. The food- 

 catching organs or tentacles have increased in number. 

 Nutting 1 has given figures of another species of Podo- 

 phrya (probably P. compressa) showing how the young 

 embryo after becoming attached, develops a few tentacles 

 at first, which increase in number with the growth of the 

 animal. Besides the tentacles there are sucking tubes 

 which broaden out at the end after the fashion of a 

 sucker (see fig. 3). The prey is caught by the tentacles 



iAmer. Nat., XXII, 1888. 



