104 SYNOPTIC COLLECTION. 



The adult Hydra (No. 126, natural size; PI. 127, 

 fig. i) has a tubular body with a mouth at the end of an 

 oral cone. At the base of the latter is a circle of tenta- 

 cles. These never possess the primitive character of 

 solidity but are from the first hollow prolongations of the 

 body wall. This mature form buds into other hydras 

 (No. 126) ; it also reproduces sexually. PI. 127, fig. 2, 

 represents the Hydra, enlarged, with a swelling which is an 

 ovum or egg, and nearer the tentacles are two smaller 

 swellings that are sacs containing the spermatozoa. This 

 animal never buds a medusa; in fact, no medusoid char- 

 acters have been observed in the embryo, and for this 

 reason some naturalists maintain that the Hydra cannot 

 be a reduced form. The difficulty of explaining the non- 

 existence of medusoid characters in the young, howevef, 

 is not so great as that which the advocates of the primi- 

 tive character of Hydra find in explaining the existence of 

 the chitinous sheath in the embryo. There are forms in 

 other classes of the animal kingdom which have become 

 so extremely reduced as to lose even in their embryo- 

 logical development the characters of their previously 

 differentiated condition, and it seems probable that the 

 Hydra can be numbered among these forms. 



HYDROPHORA. CAMPANULARIAE. 



The Campanulariae are represented by Obelia (No. 

 128), Tima (No. 129), Gonionemus (No. 130), and 

 Gonothyraea (No. 131 ; PI. 132). In Obelia we have a 

 complicated structure and life history. Its egg develops 

 into a parenchymella which becomes attached. It now 

 forms a "star-shaped root" or hydrorhiza from which the 

 first zoon is budded ; afterward other zoons bud out from 

 it and a colony is produced. Here, as in Turritopsis of 

 the Tubulariae, we have a precocious embryo and an 

 alternation of generations between the parenchymella 

 and hydra stages. 



