HYDROIDA II 



body of the polyp. In external habitus, the polyps of these families thus markedly resemble those of 

 Eudendriidcc, and the similarity in point of internal structure is equally remarkable. The endoderm 

 of the peristome, or the proboscis, is quite predominantly composed of indifferent cells, while the 

 o-astral part has a fairly homogeneous endodermal covering of digestive cells. Owing to the devia- 

 tion of the polyps in external habitus, I have in a former work, (1909 p. 133) marked off these two 

 families as a separate sub-order, Prohoscoidea. Considering the mentioned features of organisation, 

 however, in the light of what has been stated above, it will hardly be correct to retain this sub- 

 order; but it forms a special family series, ranking with the others, where the thecaphores as a whole 

 must be said to fall, as will be further referred to in the following. 



A much disputed group is formed by the genus BonncvicUa, which I formerly (1909 a) noted 

 off as a separate family. Kiihn (1913 p. 252) inclines to the view that the genus must be referred 

 to LafoeidcE^ whereas Nutting, in his latest work (1915 p. 94) retains the family of BonnevieUidm. 

 Such description as has been given of the anatomical conditions in this species is as a matter of fact 

 not sufficient fully to elucidate the relationship of the group, and Nutting's explanation, in which 

 he considers that my previous observations may be confirmed, still lacks the essential point required; 

 to wit, the development of the so-called "veloid". A comparison with the remaining genera has led 

 me rather to incline towards Kiihn's theory, that the formation in question can hardly be regarded 

 as altogether ectodermal, the inner layer being probably rather an extreme development of the indiffe- 

 rent cells of the endoderm. If this supposition prove correct — a point which can only be determined 

 by study of the development of the polyps — the position of the genus will nevertheless still be 

 doubtful; most of the facts would then seem to lead towards their inclusion as a high form of devel- 

 opment of CampanulariidcF \ we may, however, also with good reason suppose the origin to be in 

 Lafoeida;. Having no suitable material for further study of these questions available, I do not pur- 

 pose here to enter upon further theoretical discussion as to the systematic position of the genus. I 

 would merely point out once more, that the interpretation of the peristome as an ectodermal gullet 

 is doubtful in the highest degree. 



On summing up the above anatomical data, we find that the thecaphore hydroids fall into 

 four main groups or series of families, which are of great interest from a phylogenetic point of view. 

 The most primitive family series is that of Hebellitta, with its conical proboscis and with homogeneous 

 gastral endoderm. This group, which embraces the families La/oeidcF, and Canipamilinida:^ exhibits a 

 marked affinity with the athecate family of Bougainvilliida'^ and probably originated from the same. 



From Hebellina again, two new family series are derived, viz: on the one hand that of Hale- 

 citna^ with the families HalcciidcB, Plumulariida^ and Aglaop/ieniidce, and on the other, the series Ser- 

 iulariina, with the families of Syntheciidm and Serhilariida:. Haleciina is characterised by its bipartite 

 gastral portion, which is divided into a fore-stomach (probably non-digesting) and a digestive basal 

 part. This division is, especially in certain primitive Ilalccmni species, still but slightly pronounced, 

 thus giving the transition from Hebellina. In Serlularndac, on the other hand, the basal, one-sided blind 

 sack is developed as a storage chamber; the transition from Hebellina is here represented by forms such 

 as Thyroscyphus, where the partition of the gastral parts into the divisions named is still barely indicated. 



An exceptional position is that of the Proboscoida series, with club-shaped proboscis. The 



