742 PROF. A. DENDY AND MR. R. W. H. ROW ON 



Family 6. SYCETTID^ Dencly [1892 B]. 



Diagnosis. Flagellate chambers elongated, arranged radially 

 around a central gastral cavity, their ends projecting more 

 or less on the dermal surface and not covered over by a 

 continuous dermal cortex strengthened by tangential dermal 

 spicules. Tubar skeleton articulate, with subgastral sagittal 

 radiates. Collared cells usually confined to the radial 

 chambers in the adult, and probably always with apical 

 nuclei. 



The Sycettidse, in our opinion, form the starting point of a 

 distinct line of evolution, embracing the great majority of the 

 recent heterocoel Calcarea. This view is supported by the fact 

 that in all the 44 species of Sycettidfe, Hetei-opiidse, Grantiidag, 

 Amphoriscidse and Lelapiidfe in which it has been determined,^ 

 the position of the nuclei of the collared cells is apical. 



It would appear therefore that this line of evolution must 

 have originated from homoccel ancestors with apical nuclei. Such 

 forms are known to occur amongst the simple {i. e. non- reticulate) 

 species of Leitcosolenia, as for example L. hicasi and Z. bella. 



Why the transition from the simple homocoel to the radial 

 heterocoel condition originally took place we can only guess.^ 

 That it was by the outgrowth of radial buds is clearly indicated, 

 however, by the ontogeny of the genus Sycon (compare Schulze 

 [1875]). No real intermediate forms are known, however, for we 

 can no longer consider that Dendya is on the same line of 

 ancestry, while von Lendenfeld's Homoderma sycdndra is already 

 a highly specialised Sycon with the typical Sycon skeleton, and 

 differing from other Sycons only in the persistence of the collared 

 cells in the central gastral cavity. According to Minchin the 

 Sycettidse have arisen from his homocoel family Leucosoleniidae, 

 but we have already endeavoured to shew that in the present state 

 of our knowledge it is impracticable to distinguish this family 

 from his Olathrinidse. 



The presence of sagittal radiates in which the oral angle is 

 wider than the lateral angles appeai-s to be a very constant 

 character of the Sycettidge and their derivatives. It would seem, 

 further, that the occurrence of such spicules in the articulate 

 tubar skeleton is correlated Avith the development of very definite 

 radial tubes, but it is extremely difficult, at any rate in practice, 

 to distinguish such spicules individually from equiangular 

 radiates which have become sagittal by bending of the oral rays. 

 The presence of subgastral sagittal radiates is, however, especially 

 characteristic of this line of evolution. The oxeote type of 

 spicule also tends to assume more importance in the skeleton of 

 this family than in the Leucascidte and Leucaltidpe, in which 

 families it but rarely occurs. 



