6, THE VOYAGE OF H.M.S. CHALLENGER. 



discus, apparently because the polypides are not restricted and regulated in their labours 

 by the contractile stalk, but are free to wander throughout the coencecium, and to add 

 layer upon layer to strengthen their protective investment. The buccal shield is in all 

 probability the chief secreting organ, the great sheets of membrane, consisting of the 

 secretion hardened in sea- water, being formed by its agency, but the shape of the spinous 

 processes or fimbrise suggest some other assistance, such as might be obtained from the 

 enlarged and. glandular tips of the plumose arms. 



While there can be little question that the protective house of Cephalodiscus differs 

 materially from the thickened cuticle of the posterior region of the polypides in the 

 ordinary Polyzoa, and which collectively is termed zocecium, yet it seems unnecessary to 

 complicate the subject by the introduction of new nomenclature.^ The term coencecium, 

 instituted by Professor AUman, which was used in the preliminary description, points 

 to an obvious character, and gives that amount of significance which it is always well to 

 preserve if possible in scientific terms. The term tubarium, proposed by my friend 

 Professor Lankester, is very appropriate in the case of Rhahdopleura, but does not apply 

 to the condition in CephalodisciLS. in which the common abode of the polypides is more 

 aptly indicated by the already existent term coencecium. 



It is not a matter for surprise that creatures so minute should secrete so conspicuous 

 a home for themselves, or that it should assume the algoid or zoophytic outline, especially 

 when the productions of sponges and other forms are remembered, or when we reflect 

 that even a transparent structureless fluid inside a smooth capsule (as in the Nemertean 

 stylet-pouch) can produce, in countless examples of each species, precisely the same form 

 of solid crystalline stylet. The enlistment of numbers in the present ease supplies any 

 deficiency likely to arise from minute size. The secretions, indeed, both of this form 

 and Rhahdopleura, are most interesting, and indicate a degree of skill and persistence of 

 pattern quite as marked as in much more elevated types. The condition in Cephalodiscus 

 is perhaps the more striking of the two, on account of the perfect freedom of the polypides, 

 the spinous processes or fimbriae of the surface, and the numerous anastomoses of the 

 coencecium. The peculiar shape of the latter, moreover, has probably been found to be 

 that best adapted for the preservation of the animals, by its resemblance to seaweeds or 

 allied structures in the neighbourhood, on the one hand, and on the other, by its afi'ording 

 complete aeration, abundant supply of food, and security to the little architects and 

 their delicate plumes. 



II. Polypides. 



The rounded cavities and canals of the semitransparent coencecium contain numerous 

 opaque masses (the polypides) and large ova slightly attached by their peduncles. The 

 former often occur in groups, each individual, however, except in the case of buds, 



' E... Edicularium. 



