518 GENERAL STRUCTURE OP ALCYONIAN POLYPES. 



1088. The ova or germs are developed in the substance of 

 the membranous folds, or from the lining of the canal. They 

 grow like seeds in a seed-vessel ; at first appearing as little pro- 

 tuberances from the plane surface ; then acquiring a distinct 

 globular form, and remaining attached by a little stalk ; and 

 finally, being liberated by the separation of this pedicle. They 

 then make their way outwards, by passing into the stomach 

 through its lower aperture; and finally escape by the mouth. 

 Like the gemmules of other Polypes, they consist in this state 

 of a membranous bag including fluid; and they are covered with 

 cilia, by the action of which they move freely through the water 

 for some time before fixing themselves, though not with the same 

 rapidity as the ova of some other Polypes. They are usually 

 coloured with great vividness ; and, during their motions, they 

 often contract themselves and alter their form. 



1089. Very little is known of the general habits of these 

 Polypes ; but they probably differ but little from those of the 

 groups already described. We may pass on, therefore, to con- 

 sider the chief subdivisions of this Order, which includes many 

 very interesting and well-known forms. These subdivisions are 

 principally founded upon the nature of the polypidom ; and it is 

 desirable, before proceeding to describe them, that the relation 

 of the parts of which the animals are composed should be clearly 

 understood. Delicate as is the membrane of the Polypes them- 

 selves, it may be distinguished, like that of the Sertularians 

 ( 1052) into two layers. Of these, the outer one is continuous 

 with the general envelope of the whole mass ; whilst the inner 

 one lines the canals which ramify through it. But instead of 

 these two membranes being in contact, as are the horny sheath 

 and the lining of the tubes in the Sertularians, a thick mass 

 of flesh is interposed between them ; and it is usually in this 

 flesh, and not in either of the membranes, that the hard deposit 

 takes place, which gives firmness and support to the general 

 structure. 



1090. The family we shall first consider, includes th( 

 species which have a spongy polypidom, of which the Al 



