PORIFERA. 809 



specialised ; (7) the typical collared character of the endoderm cells is a unique 

 feature ; but it must not be forgotten that in the planula or larva of Oscarella lobu- 

 laris the ectoderm cells are also collared ; (8) the group is one of extreme antiquity. 



A small group, the Gastreadae of Haeckel, of doubtful affinities may be 

 mentioned here. They are marine sponge-like organisms with a body-wall com- 

 posed of an ectodermal syncytium (? mesoglea + ectoderm) and a ciliated collared 

 endoderm, supported by a skeleton of sponge-spicules, Radiolarian shells, &c., 

 according to habitat. Pores are absent. There are two sub-groups : (i) Physe- 

 maria, body-wall thin, solid, a single osculum, with four genera, two solitary, 

 Haliphysema, Gastrophysema, and two colonial (?), Dendrophysema, Clathrophysema 

 (2) Caementaria, body-wall thick, traversed by gastral tubes lined wholly or in part 

 by ciliated endoderm, mostly deep sea, with four genera, Caementascus with a 

 single osculum, Caementoncus, Caementissa and Caementura with several. Ova have 

 been seen in several instances : filiform spermatozoa and an invaginate gastrula in 

 Gastrophysema dithalamium. 



The group requires a careful re-examination. Haliphysema Tumanouriezii, 

 which was originally described as a sponge by Bowerbank, has been proved 

 to be one of the Protozoan Reticularia by Kent, Lankester and Mobius, as it 

 was surmised to be by Carter. Moreover, it is identical with Carter's Squamulina 

 scopula= Gastrophysema scapula according to Norman, which Carter supposed 

 originally to be a Reticularian. Haeckel, however, states that he has himself never 

 seen the Squamulina in question, and classes it among Physemaria on the strength 

 of his own views relative to Carter's observations. A fate similar to that of H. Tu- 

 manowiczii has betaken Wagnerella borealis from the White Sea, described by 

 Merejkowsky as a sponge, but now held to be one of the Protozoan Heliozoa, 



Neue Gastraeaden, Haeckel, SB. Jen. Ges. 1883, pp. 84-9. Physemaria, 

 Id., J. Z. xi. 1877. Haliphysema Tumanowiczii, Saville Kent, A. N. H. (5), ii. 

 1878 ; Ray Lankester, Q. J. M. xix 1879 ; Mobius, Beitrage zur Meeresfauna 

 der Insel Mauritius, etc., Berlin, 1880, pp. 72-5, Pis. i, ii, i. On Haliphysema 

 and allied forms, Norman, A. N. H. (5), i. 1878. 



The Porifera are classified as follows : 



I. Calcarea= Caldspongiae : skeleton composed of calcareous spicules. 



1. Homocoela : gastric cavity lined in all parts by collared cells ; three families, 

 (i) Asconidae Ascones : gastric cavity flask-shaped and body-wall plain, Leucosolenia 

 =Ascetta, Ascandra, &>c. ; (ii) Homodermidae : body- wall produced into radial tubes 

 similar to those of Syconinae (infra), Homoderma Sycandra ; (iii) Leucopsidae : a 

 large amount of mesoglaea imbedding ' mouthless Ascon-persons,' a large pseudo- 

 gaster and pseudostome, Leucopsis pedunculata. 



2. Heterocoela : endoderm differentiated into two forms of cells, pavement 

 epithelium cells lining a central portion of the gastric cavity, and collared flagellate 

 cells restricted to limited regions either radial tubes or ampullae ; four families, (i) 

 Syconidae= Sy cones, body-wall produced into radial tubes or cones with sensory cells 

 round the orifices of the inhalent canals, with three sub-families (pp. 793-4), Syconinae, 

 Sycon= Sycetta, Sycandra, Sycortis, Sycaltism part, Uteinae, e.g. Ute, Grantessa, &c., 

 Grantinae, e. g. Grantia ; (ii) Sylleibidae, with a complicated inhalent canal-system, 

 and sac-like ampullae either placed radially to main axis of the sponge and con- 



