8/0 SPONGES AND ZOOPHYTES 



a recurrence of the operation, this, as observed by Mr. Lister, 

 being repeated at intervals of eighty seconds. Besides the foregoing 

 movement, a regular flow of fluid, carrying with it solid particles of 

 various sizes, may be observed along the whole length of the stem, 

 passing in a somewhat spiral direction. It is worthy of mention 

 here that when a Tubularia is kept in confinement the polype-heads 

 almost always drop off after a few days, but are soon renewed 

 by a new growth from the stem beneath ; and this exuviation and 

 regeneration may take place many times in the same individual. 1 



It is in the families Gampanulariida and Sertulariida (whose 

 polyparies are commonly known as 'corallines') that the horny 

 branching fabric attains its completest development, not only afford- 

 ing an investment to the stem, but forming cups or cells for the 

 protection of the polypites, as well as capsules for the reproductive 

 gonozb'oids. Both these families thus belong to the sub-order Thecata. 

 In the Gampanulariida the polype-cells are campanulate or bell- 

 shaped, and are borne at the extremities of ringed stalks (fig. 659, c) ; 

 in the Sertulariida, on the other hand, the polype-cells lie along the 

 stem and branches, attached either, to one side only, or to both sides 

 (fig. 662). In both the general structure of the individual polypes 

 (fig. 659, B, d) closely corresponds with that of the Hydra ; and the 

 mode in which they obtain their food is essentially the same. Of 

 the products of digestion, however, a portion finds its way down into 

 the tubular stem, for the nourishment of the general fabric ; and 

 very much the same kind of circulatory movement can be seen in 

 Campamdaria as in Tubularia, the circulation being most vigorous 

 in the neighbourhood of growing parts. It is from the ' ccenosarc ' 

 (fig. 659, f) contained in the stem and branches that new polype- 

 buds (b) are evolved ; these carry before them (so to speak) a portion 

 of the horny integument, which at first completely invests the bud ; 

 but as the latter acquires the organisation of a polype, the case 

 thins away at its most prominent part, and an opening is formed 

 through which the young polype protrudes itself. 



The origin of the reproductive capsules or ' gonothecre ' (e) is 

 exactly similar, but their destination is very different. Within 

 them are evolved, by a budding process, the generative organs of 

 the zoophyte ; and these in the Campanulariida may either develop 

 themselves into the form of independent .medusoids, which com- 

 pletely detach themselves from the stock that bore them, make their 

 way out of the capsule, and swim forth freely, to mature their 

 sexual products (some developing sperm-cells, and others ova), and 

 give origin to a new generation of polypes ; or, in cases in which 

 the medusoid structure is less distinctly pronounced, may not com- 

 pletely detach themselves, but (like the flower-buds of a plant) expand 

 one after another at the mouth of the capsule, withering and drop- 

 ping off after they have matured their generative products. In the 

 fiertulariida, on the other hand, the medusan conformation is wanting, 

 as the gonozooids are always fixed ; the reproductive cells (fig. 662, #), 

 which were shown by Professor Edward Forbes to be reallv meta- 



1 The British Tulnlariida form the subject of a most complete and beautiful 

 monograph by the late Professor Allman, published by the Kay Society. 



