118 



COLLEGE ZOOLOGY 



stimuli and for conduction of impulses from 

 one part of the body to another. Coelen- 

 terates are generally sensitive to light inten- 

 sities, changes in temperature, mechanical 

 stimuli, chemical stimuli, and gravity. Re- 

 production is both asexual, by budding and 

 fission, and sexual, by means of eggs and 

 spermatozoa. 



Obelia— a colonial hydroid 



Obelia (Fig. 57) lives in water up to 240 

 feet in depth, along our eastern coast from 

 Long Island Sound to Labrador, on the 

 Pacific Coast, and other parts of the world. 

 If you can imagine a hydra budding with- 

 out the buds detaching from the parent 

 body, and then imagine these new individ- 

 uals specializing for certain functions as 

 feeding and reproduction, it will be easy to 

 understand the development of Obelia. It is 

 attached to the substratum by a rootlike 

 mass ( hydrorhiza ) , from which arise up- 

 right branches (hydrocauli). Hydralike feed- 

 ing members (hydranths) and reproductive 

 members (gonangia) are given off from 

 the hydrocaulus, as shown in Fig. 57. The 

 soft parts are protected by a cellophanelike, 

 chitinous covering (perisarc), which is 

 ringed at intervals and expands around the 

 hydranths to form the hydrothecae and 

 around the gonangia to form the gonothe- 

 cae. The soft parts ( coenosarc ) are attached 

 to the perisarc by small strands. 



The hydranths resemble the hydra some- 

 what in structure and function, but these are 

 specialized for feeding only. The tentacles, 

 however, are solid and about 30 in number. 

 The central axis ( blastostyle ) of the gonan- 

 gium gives rise to buds that develop into 

 medusae; these escape through the opening 

 in the end of the gonotheca. The free- 

 swimming medusae produce either eggs or 

 spermatozoa. The fertilized egg (zygote) 

 develops into a ciliated, free-swimming larva 

 (planula) which soon becomes attached to 

 a stone and grows into a polyp type of col- 

 ony that reproduces asexually by budding. 



Metagenesis 



The alternation of a generation that re- 

 produces only asexually by division or bud- 

 ding, with a generation which reproduces 

 only sexually by means of eggs and sperms, 

 as in Obelia, is known as metagenesis. The 

 polyp and medusa stages are not equally 

 prominent in all Hydrozoa; for example, the 

 medusa in some species is degenerate or 

 inconspicuous, as in Obelia, whereas in other 

 species the polyp generation is only slightly 

 developed, as in Gonionemus. 



Gonionemus— a hydrozoon 

 medusa 



Gonionemus is a jellyfish (Fig. 58), com- 

 mon along the eastern coast of the United 

 States. It measures about Vz inch in diam- 

 eter, and bears around the margin from 16 

 to 80 or more hollow tentacles which bend 

 at a sharp angle near the tip. The gonads 

 are brown. The convex or aboral surface is 

 the exumbrella; the concave or oral surface, 

 the subumbrella; this is partly closed by a 

 perforated membrane, the velum. Water is 

 taken into the subumbrellar cavity and is 

 then forced out through the central opening 

 in the velum by the contraction of the body; 

 this propels the animal in the opposite di- 

 rection, a sort of jet propulsion. Hanging 

 down into the subumbrellar cavity is the 

 manubrium with the mouth at one end sur- 

 rounded by 4 frilled oral lobes. The mouth 

 leads to the gastrovascular cavity in the mid- 

 dle of the bell, where 4 radial canals ex- 

 tend to a ring canal which lies near the mar- 

 gin of the umbrella. 



The cellular layers are similar to those in 

 the hydra, but the mesoglea is extremely 

 thick and gives the animal a jellylike con- 

 sistency. Suspended beneath the radial 

 canals are the sinuously folded reproductive 

 organs or gonads. One individual produces 

 either eggs or spermatozoa; therefore Go- 

 nionemus is dioecious. A ciliated planula de- 

 velops from the fertilized egg; it is at first 



