COMPENDIUM. 277 



We find no sensible medium whereby the benefit derived from the 

 sustenance of the ascidian hydra is transmitted to the polyparium, though 

 their anatomy is more explicit. The like may be said of the pectinate 

 hydrae, whose anatomy, on the other hand, is less explicit. 



Were we to view the hydra comprehensively, we should find room for 

 several important distinctions. Thus, there are noted characters derived 

 from the tentacula, in being muricate and ciliated. Those of the Helian- 

 thoids, unlike the rest, are smooth. 



Muricate tentacula belong to the naked hydra of the Tubularian and 

 Sertularian zoophytes : Pectinate to the hydra of the Lobularia, Virgularia, 

 and Pennatula ; the Actinias have smooth tentacula ; those of the Flustra, 

 Alcyonium, Cristatella, and other lunate zoophytes, are ciliated ; but the 

 animals belonging to them are distinguished by features equally promi- 

 nent, and by more complex organization than the rest. 



The number of tentacula is rarely a positive feature. There is one 

 row in the naked Hydra, commonly three in the Actinia ; two in the Tu- 

 bularia, one in the other fistulous zoophytes : and they are arranged in dif- 

 ferent stages in the Coryna squamata. 



In the lunate ascidian hydrse, they are in a single encircling row, 

 disposed in a horse-shoe or crescent form, though sometimes referred to 

 as in a double row. But the row is truly single. 



The number of tentacula is exceedingly variable in many zoophytes ; 

 as it is in the simple hydra. Eight belong to all the pectinate hydrae of 

 the carnose genera, or asteroid zoophytes ; but this number distinguishes 

 only some species of the other genera ; as the Sertularia arcta of the hy- 

 draoids. The ascidian hydra?, also, of the genus Valkeria, have eight ten- 

 tacula. There is scarcely any other fixed number ; for in the rest they are 

 so irregular, that they will be found advancing from ten to above an hun- 

 dred. These organs discharge the office of fingers or hands, seizing the 

 prey of the animals, and conveying it to the mouth, where it is always 

 devoured entire. Many tentacula, to be subsequently acquired, are de- 

 ficient in most of the younger zoophytes. Their successive evolution is 

 beautifully illustrated in the Hydra tuba, where, commencing with two, 

 they may ultimately exceed thirty. This successive evolution continues 



