REPORT ON THE HEXACTINELLIDA. 493 



teristic that one can hardly find a more striking proof of close relationship. Of less 

 importance, though in detail often not without striking peculiarities, are the characters 

 which distinguish the four living families of Scopularia. Thus, for example, the family 

 of Melittionidae, though including only the single genus AphrocaUistes, with a few 

 species, is so sharply defined and removed fi'om the other families by the hexagonal, 

 prismatic, honeycomb-like radial perforations of its fiat dictyonal framework, that one 

 must assume a long, independent, ancestral series, that is to say, a somewhat early 

 separation of the twig from the common branch of the Scopularia. The characters of 

 the primitive portion of this branch appear to me to be represented in their least 

 modified form by the family of the Euretidae, where the structure is comparatively 

 simple and slightly ditferentiated, especially in regard to the aflferent and efferent canals 

 penetrating the body-wall. For while in the Euretidse the afferent and efferent canals, 

 which traverse the thin wall of the tubes forming the entire sponge, are quite short and 

 usually sack-shaped, of simple uncharacteristic form, the efferent canals of the Melittionidse 

 are straight, hexagonal prisms, those of the Coscinoporidee straight, narrow, and usually 

 long, alternating funnels penetrating the body-wall at right angles, and those of the 

 Tretodictyidae, finally, are of irregular course. And as to the other Hexasterida 

 which contain no uncinates, the family of the Masandrospongidaj, which flourished in 

 the Cretaceous period, is distinguished by the structure of the body, which consists of a 

 system of anastomosing, meandering, thin-walled tubes, with an interjacent system of 

 anastomosing canals. 



But while these Mseandrospongidse have long since become true Dictyonina, the 

 other families of Hexasterophora, without uncinates, have either entirely preserved the 

 Lyssacine character, or they nearly approach the Dictyonine type by the more or less 

 marked soldering of the larger spicules into a connected framework, which develops 

 with increasing age. Of the three families — Euplectellidse, Asconematidse, and Rossel- 

 lidse, the first has a markedly separate position, due to the hexradiate character of the 

 spicules supporting the skin (hyjiodermalia), while the others exhibit only pentact hypo- 

 dermalia, with an internal radial ray. While in the Asconematidse autodermal piuuli 

 are also developed within the skin, giving to this Hexasterid family a very peculiar 

 character somewhat resembling that of the Hyalonematidse, in the Rossellidse such 

 autodermalia, with a freely projecting fir-tree-like ray, do not occur. It is difficult, if 

 not indeed impossible, to determine, with any certainty, the relationship of these three 

 allied families. It is obvious that the Euplectellidse, both in their relatively simple, 

 saccular or tubular form, and also in their hexradiate dermal sjjicules, have retained 

 primitive characters, so that one need not wonder that very simple related forms are 

 found at a comparatively early period. On the other hand, both in the parenchyma 

 of several species, such as Dictyoccdyx gracilis, and also in the extremities of 

 the dermal projections of many genera, e.g., Euplectella, Twgeria, Walteria, &c., 



(ZOOL. CHALL. EXP. — PART LIII. — 1887.) Ggg 63 



