HARDWICKE'S SCIENCE-GOSSIP. 



5i 



According to Dr. Allman, the polyzoa have three 

 distinct modes of reproduction. By buds or gemma?, 

 by true ova, and by free locomotive embryos. 



The gemmoe or buds are developed on the body of 

 the polypides : this always happens whenever the 

 cells are in mutual apposition. If the cells are dis- 

 tinct they are developed from the connecting stem or 

 stolon, as in the recent Laguncula reptans. 



"The best examples" of the former mode "are 

 furnished by the Flustra and their allies. From a 

 single cell of the Flustra five such buds may be sent 

 off, which develop themselves into new polypides 

 around it ; and these in their turn produce buds from 

 their unattached margins, so as rapidly to augment 

 the number of cells to a very large amount. To this 

 extension there seems no definite limit, and it often 

 happens that the cells in the central portion of the leaf- 

 like expansion of a Flustra are devoid of contents and 

 have lost their vitality whilst their edges are in a state 

 of active growth."* 



Since this was written the " dead cells " have formed 

 the subject of many an excellent paper by Claparede, 

 Smitt, Nitsche, and Hincks. The cells are not dead 

 in a true and literal sense, for they often contain black 

 or brown spots, supposed by Ellis (1755) " to be the 

 remains of the animals once inhabiting these cells." 

 These dark bodies are supposed — and their history 

 has been accurately traced by Hincks — to be "germ 

 capsules," and these may be characterised, if not as a 

 fourth, at least as a very peculiar method of repro- 

 duction. This view, however, is opposed to that of 

 Claparede who considered the " dark bodies to be the 

 result of retrogressive metamorphosis of the original 

 polypides which, under certain circumstances shrink 

 back into this rudimentary condition, passing through 

 the same stages in their decline as in their progress 

 towards maturity, but in an inverse order." t 



Reproduction by ova is the result of impregnation 

 of the ova with the spermatozoa. Both the male and 

 the female particles are developed within the same 

 polypides, only situated in different parts of the body.J 

 The embryo is first a hollow sphere, a layer is then 

 thrown off from the surface at the same time that an 

 opening is made in the wall of the sphere ; a second 

 sort of little sphere is thus formed within the first, and 

 here little polypides are gradually developed. § This 

 development often takes place within the body of the 

 parent, and their final discharge is by an opening 

 situated beneath the tentacular circle. 



I do not take into this account all the facts that 

 have been promulgated respecting the reproduction of 

 polyzoa by buds and by fertilised ova. A good paper 

 by Nitsche, on the mode of reproduction of Flustra 

 membranacea, is to be found in the " Quarterly Journal 



* Dr. Carpenter, 1868. 



t Hincks's " Contribution to the History of Polyzoa." 

 % See Dr. Carpenter, p. 578-9, "Quarterly Journal of the 

 Microscopical Society." vol. xiii. 

 $ Dr. Ord, M.D. 



of the Microscopical Society," vol. ii. It is true, how- 

 ever, that not all the polypes are equally reproductive. 

 In recent polyzoa, and in all probability in the fossil 

 also, there were distinct centres of reproductive energy. 

 These are the oocecia or the ovicells of Busk and 

 others. In some of the polyzoa the ovicells are 

 separated from the ordinary cell structure and are 

 developed in the axils of the zoocecia, or else by an 

 inflation of the ordinary cell. Among the cyclosto- 

 matous polyzoa, the ovicells of Crisia and Crisidia 

 are thus formed in the axils. The observations made 

 on the ovicells of Idmoneidse are scanty — but in the 

 species called Idmonea gracillima, brought to light 

 during the Porcupine soundings from a depth in the 

 Atlantic at from 286 to 322 fathoms, the ovicells are 

 pyriform like Crisia. 



In Hornera frondiculata the ovicells are oblon j 

 and keeled— and in this and in several other species 

 they are dorsal : while in H. violacea the ovicells are 

 anterior either wholly or in part. They are unknown 

 in Retihornera, but in Pustulopora they are tumid. 

 In the Tubuliporidce (Alecto and Tubulipora) the 

 ovicells are represented by an uniform inflation of a 

 part of the zoarium. 



The ovarian cells of many of the Cheilostomata are 

 cells situated among the ordinary cells of the polyzoary. 

 They are known by certain characters and are easily 

 distinguished by those who are in any way acquainted 

 with the polyzoa. In the Salicornaria either a 

 conical tooth or an elongated slit marks the ovarian 

 cell. In the Membranipora, they are either triangu- 

 larly marked, deeply immersed, or large and con- 

 spicuous. In Lepralia some are peculiarly punctured, 

 or else globose. In Cellepora and Eschara they are 

 either globose or else subglobose — except in E. mom~ 

 lifera, here there are no ovicells but what answers 

 the same purpose^/?;-///*? cells, large, depressed, and 

 irregularly placed. In Melicerita the ovicells are im- 

 mersed, opening with a cresentic within the summit 

 of the cell ; while in Retepora there is either a vertical 

 slit, or a large opening in front. 



To the living polyzoa two very remarkable, but 

 minute appendages are attached. One is the avicu- 

 laria, or bird's head process ; the other is the vibri- 

 cula or whip-like spines. Much doubt exists as to 

 the real function of either of the appendages. They 

 are present in nearly all the Cheilostomatous polyzoa. 

 In the Cyclostomata these appendages are rare — 

 probably the vibricula are only found among the 

 Crisia and the Crisidea — and as the carboniferous 

 polyzoa are generally placed among the Cyclostomata 

 it would be useless therefore to seek for these appen- 

 dages. But there is strong presumptive evidence that 

 in some at least of the Fenestella and Glauconome 

 we may discover— either by inflations of the cells, or 

 by gibbous masses clustering round the cell mouths, 

 indications of one of the modes of reproduction 

 prevalent among the carboniferous polyzoa. For 

 specimens of these gibbous masses I have sought 



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