49 



Art. tg of the Rules of Nomenclature 1 ). I must admit that this procedure involves stretching 

 the Rule, but it has the advantage of making the name of the genus next to be described 

 more different from that of the present genus than would otherwise be the case. 



i. Arachnidium irregulare n. sp. (PI. III, figs 3 — 6). 



376. B. Stat. 213. Saleyer, O — 36 Metres; coral-reefs, mud, and mud with sand. (On the dermal 

 membrane of a Siliceous Sponge). 



Zooecia discrete, relatively large. Daughter-zooecia given off irregularly (hence the specific 

 name), separated from the parent by a diaphragm and at first narrow and cylindrical, then 

 expanding into a broad part, varying in form, and containing the viscera. Orifice borne on a 

 prominent, broad mammilla, situated subterminally, marked by concentric transverse lines, but 

 not angulated. Tentacles numerous. 



The present species has been found once only. The spirit containing it had almost dried 

 up on arrival. 



Hixcks (1862) stated, in characterising the genus, that "The Arachnidia may be 

 "regarded as an Alcyonidium with its cells detached from one another, and held together by a 

 "delicate thread". A irregulare conforms well with this definition: the "delicate thread" beinor 

 merely the proximal end of a zooecium. The length of this part is variable. The entire zoarium 

 is very closely adherent to the substratum, and the wide part of the zooecium is depressed, 

 ancl varies much in outline. 



The cruciform disposition of the zooecia is usually apparent in Arachnidium ; and the 

 present species shows distinct traces of it (tig. 3), accompanied by some irregularity. Thus I 

 find that in A. irregulare there is commonly 110 symmetrically placed distal daughter-zooecium, 

 but that a zooecium is given off from each of the distal corners of the parent. This may be 

 the result of a displacement of the symmetry, by which one of the lateral zooecia appears to 

 form a pair with the distal zooecium (fig. 5). In other cases (fig. 4) two zooecia are given off 

 on one side and none on the other, the distal zooecium being median in the specimen figured. 

 In some cases (fig. 3) the two lateral zooecia are not opposite one another. 



The orifice is borne on a massive mammilla, which is about half as broad as the attached 

 part of the zooecium, from which it rises nearly at right angles. The mammilla shows no tracé 

 of angulation, but is circular in transverse section, and is marked by conspicuous annulations. 

 The "collar" which is characteristic of the more typical Ctenostomata is not well showt) in the 

 preparations, but it appears to be present and of considerable size. 



The tentacles are rather short and are numerous: — probably about 26 — 30. The 

 oesophagus is of moderate length (fig. 6) and the stomach and its caecum are large. In some 

 of the zooecia there appear to be several large embryos, one of which, contained in an ovisac 

 provided with radiating muscles, is shown in fig. 5. The vestibule has persisted, but the rest 

 of the polypide has degenerated. 



In view of the variability of the proximal part of the zooecium it is not easy to state 



teA 



1) IXe Congres, Int. Zool., Monaco (1913), 1914, p. 899. 



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SIBOGA-EXPEDITIE XXVIII. /. 



