I30 CCELENTERATA OH STINGING-ANIMALS. 



reproductive "persons" of fixed and asexual hydroid colonies, and that 

 they produce ova and spermatozoa, from which develop embryos that 

 start new zoophyte colonies. Thus the life-history illustrates alternation 

 of generations. 



If this were the case in all HydromedusEe, there would be no diftculty 

 in summing up the characters of the order. But it is not so. The puzzle 

 is that many hydroid colonies do not give off medusoid bells, but have 

 permanently attached and more or less reduced reproductive |' persons.' 

 Besides, there are many free medusoids, anatomically similar to the 

 detached buds, but without any connection with fixed zoophyte colonies. 



(a) There are hydroid colonies with nutritive Hyiira-Wke persons and 



other peculiarly modified reproductive persons or gonophores 

 which are not set free as such. Such are the Hydrocorallin^ 

 (millepores), and many zoophytes, especially Plumularians and 

 Sertularians. 



(b) There are hydroid colonies with nutritive Ilydra-Vike persons and 



other peculiarly modified reproductive persons which are set firee 

 as sexual swimming -bells or medusoids. Such are many Tubular- 

 ians and Campanularians. 



(c) There are free medusoid animals, anatomically similar to the 



liberated bells above-mentioned, which have, however, no con- 

 nection with hydroid colonies, but produce embryos which grow 

 into other medusoids. These are called TrachymedusEe, and 

 include two sub-orders, viz. , the Trachomedusae, e.g. , Geryonia and 

 Carmarina, and the Narcomedusfe, e.g. , Cunina and y^ginopsis. 

 For our present purpose, it is sufficient to regard the various kinds 

 of Hydroids as " compound-Hydrse," supported by an external skeleton, 

 with division of labour in the colony to this extent at least that the 

 nutritive persons are different from the reproductive. 



The Medusoid Type. 



The medusoid is like a bell ; the mouth is at the end of a prolongation 

 (manubrium) which hangs like the clapper; the margin of the bell bears 

 solid or hollow tentacles beset with stinging-cells ; and a flap or shelf 

 narrows the opening of the medusoid as the mouth of a bell would be 

 narrowed if the margin were bent inwards. This flap {velum or 

 craspedon) is one of the anatomical characters which distinguish medusoids 

 (Craspedota) from jellyfish (Acraspeda or Scyphomedusse) like Cyanea. 



The surface of the medusoid is covered by ectoderm, with stinging- 

 cells on the tentacles, muscle-cells on the manubrium, nervous cells around 

 the margin ; a ciUated endoderm lines the food-canals and extends into the 

 tentacles ; and an intermediate middle stratum is especially developed on 

 the upper (ex-umbrellar) side of the medusoid as a gelatinous watery 

 stuff, crossed by slim fibres, — being literally a middle jelly or mesogloea. 



Small organisms stung by the stinging-cells of the tentacles and 

 gathered by the lips of the mouth, pass up the manubrium into the central 

 cavity of tfie dome, are wafted by cilia down a number of radial canals, 

 may reach one of the canals running around the margin, and are sooner 

 or later absorbed by the endoderm cells, and subjected to intra-cellular 

 digestion. 



