82 



LOWER INVERTEBRA TES. 



young come fortli in an aetinula condition, jiresenting distant resemblances to a jelly- 

 fish. The body is long and sun-ounded by a single circle of tentacles. This larva 

 soon becomes attached and then develops into a form like the parent. 



Many of the stnall s])iral shells found in the shallow salt-water just below the 

 water's edge, are found to bo inhaliitod by hermit crabs, which travel about very 



actively by protruding their legs 

 from the ajierture of the shell. 

 On the backs of many of these 

 shells is what appears to the 

 eye, a white, delicate, mossy 

 growth, covering most all of the 

 shell, excepting that part M'hich 

 drags on the bottom as the crab 

 travels. Under the microscope, 

 this mossy growth proves to 

 be a colony of very beautiful 

 hydroids named Hydractinia. 

 They live in colonies, but in- 

 stead of forming a colony by 

 branching in the ordinary way, 

 the hydrorhiza, or part which 

 attaches the colony, spreads out 

 farther and farther, and sends nj) 

 more and more buds, each one 

 of which becomes a zooid, but 

 which does not bud and is not 

 covered by chitin. The hydro- 

 rhiza is covered by a layer of 

 chitin, and at in-egular intervals 

 the chitin is developed into a 

 large projecting spine. The 

 zooids are very contractile, and 

 when withdrawn to their utmost, 

 the hard chitinous spines pm- 

 ject slightly beyond and jirotect 

 them. Examining carefully the 

 zooids of IL/dractinia it is found 

 that there are the ordinary feed- 

 ing zooids, the reproductive 

 zooids, male and female, and a 

 third kind which are destitute 

 of true tentacles, have very slender, much elongated bodies, and are powerfully 

 armed with strong batteries of cnidocells with which they perform their duty of 

 protecting the colony. From a fertilized egg of Hydractinia is develojied a planula, 

 which in time gives rise to a Hydractinia colony. There are a large number of 

 jelly-fishes known, which, from their structure, are classed among the GymnolilastcM, 

 although nothing is known of their attached hydroid condition, or even if they jiass 

 throuarh sueli a stage. 



-Bydr, 



nutritive, h, fen 



