CELL AGGREGATION AND DIVISION OF LABOR 



97 



much branched cosnosarc (interior cellular portion) bearing at the ends of 

 the branches flower-like zooids, called polyps or hydranths. The polyps 

 are each provided with a hypostome (see figure) and a circlet of tentacles. 

 The coenosarc is surrounded by a tough cuticle or perisarc secreted by the 

 cells of the coenosarc. The colony arises from a branched root-like struc- 

 ture, the hydrorhiza, which is attached to a solid body such as a rock or log. 

 This colony is produced by budding without separation of the buds from 

 the parent. From the stalks of many of the polyps, medusce (jellyfishes) 

 are formed by budding Medusae do not resemble polyps but are bell- 

 shaped individuals (Fig. 62) which after maturity become separated from 

 the colony and swim freely in the water by means of rhythmic contractions 

 of the bell. Each medusa produces eggs or 

 sperms. The fertilization of an egg by a 

 sperm results in the formation of a ciliated 

 free-swimming embryo which eventually 

 attaches itself by one end to a rock and 

 develops into a polyp. This polyp puts 

 forth buds and thus a new colony is formed. 



Obelia forms a colony somewhat re- 

 sembling Bougainvillea. In Obelia, how- 

 ever, the medusa buds are produced by 

 budding from the stalks of certain branches 

 {blasto styles) which do not bear polyps at 

 their tips. Each blastostyle is enclosed in 

 a swollen chitinous sheath, the gonotheca. 

 Obelia is thus composed of three types of 

 individuals, two of which are sessile and 

 incapable of sexual reproduction, while the 

 other is a sexual free-swimming form. 



In Hydractinia (Fig. 63) the coeno- 

 sarcal coi'es of the branches of the hydro- 

 rhiza are fused to form a dense crust on the 

 surfaces of certain shells inhabited by hermit 

 crabs. From the hydrorhiza arise many zooids on unbranched stalks. 

 Some of the zooids are nutritive polyps, that is, procurers and digesters of 

 food; others are defensive zooids; and the members of a third class give 

 rise to sporosacs which are really abortive medusae, that is medusae in 

 which the gonads are well developed but which have lost their tentacles 

 and mouth and the means of locomotion. The sporosacs serve only the 

 function of producing eggs or sperms and must depend upon the feed- 

 ing individuals for sustenance. 



The examples of Hydrozoa chosen to illustrate colon j^ formation also 

 illustrate differentiation and division of labor. Differentiation of indi- 

 viduals has progressed to such an extent that the species exhibit pohjvior- 



7 



Fig. 63.— Portion of a female 

 colony of Hydractinia echinata. 

 P, nutritive polyp; Pi, mouthless 

 spiral polyp; g, blastostyle with 

 sporosacs; h, hydrorhyza. (After 

 Alb}ian.) 



