SPONGES 



51 



cation. 



granules ; at first they exhibit lively amoeboid movements, 

 but later pass into a resting stage. The cavity of the 

 mesoderm within which they are situated becomes lined 



Fig. 24.— Spermatozoa, a-h, Development of spermatozoa in Sycandra raph- 

 omus, highly magnifiecL ; ft, mature spermatozoa. After Polqaeff(x 792). j, 

 A spenn ball in Osmrella loiularis ( X 600) ; It, an isolated mature spermatozoon. 

 After Schulze(x 800). 



by a layer of epithelium, which may not appear, however, 

 till a late stage of segmentation. In Empongia officinalis 

 the ova occur congregated in groups within the mesoderm, 

 thus presenting an early form of ovary. The spermatozoa, 

 which also develop from wandering amoeboid cells, are 

 minute bodies with an oval or pear-shaped head and a 

 long vibratile tail (fig. 24 h). Each amoeboid cell produces 

 a large number of spermatozoa, which occur in spherical 

 clusters or sperm-balls. The heads of the spermatozoa, 

 as in the Metazoa, are produced from the nucleus of the 

 mother-cell, the tails from the surrounding protoplasm. 

 The development in detail is upon two plans. In Grantia 



Family 1. TETEACLADlDiE. — With the characters of the sub- 

 order. Examples : Theonella, Gray (fig. 21 k) ; Biscodermia, Socage; 

 *Siphonia, Parkinson. 



Sub-order 2. RBABDOCREPIDA. 



The desmas are of various forms, produced by the growth of silica 

 over a uniaxial spicule. 



Family 1. MEGAMOElNlDiE. — The desmas are comparatively 

 large. Triaenes, usually dichotriaenes, help to support the ecto- 

 some. Microscleres usually spirasters. Examples : Corallistes, 

 O. S. ; * Syalotragos, Zittel ; Lyidium, 0. S. ; * Dorydermia, Zittel. 



Family 2. Mickomorinid^. — The desmas are comparatively 

 small. Triaenes and microscleres are both absent. Examples : 

 Azorica, Crtr. ; * VerrucUna, Zittel. 



Sub-order 3. ANOMOCLADINA. 

 Desmas with a massive nucleated centrum, from which a variable 

 number of arms {sS) extend radiately (see fig. 12/). Examples : 

 Vetulina, O.S. ; *Astylospongia, Eoemer. 



Beproduction and Embryology. 

 Fresh individuals arise by asexual gemmation, both 



external and internal, by fission, and by true sexual repro- 

 duction. 

 Asexual Fission is probably one of the processes by which com- 

 multipll- pound sponges are produced from simple individuals. 



Artificial fission has been practised with success in the 



cultivation of commercial sponges for the market. Ex- 

 ternal gemmation has been observed in Thenea, Tethya, 



Folymastia, and Oscarella. A mass of indifferent sponge- 

 cells accumulates at some point beneath the skin, bulges 



out, drops off, and gives rise to a new individual. Internal 



gemmation, which results in the formation of a statoblast, 



is only known to occur in the freshwater Spongillidx. 



The statoblasts consist of a mass of yolk -bearing 



mesoderm cells, invested by a capsule, which in 



Ephydatia fluviatUis is composed of an inner 



cuticle of spongin- separated from a similar outer 



layer by an intermediate zone of amphidisks and 



interspersed protoplasmic cells. On one side of 



the capsule is a hilum which leads into the interior. 

 Their development has recently been studied by Gbtte, 



with results that confirm the conclusions of Carter (j) 



and Lieberkiihn {is)- The process commences with an 



accumulation of amoeboid cells within the mesoderm to 



form a, globular cluster; yolk granules develop within 



them, especially in those that lie nearer the centre. The 



external cells give rise to the investing capsule; some 



resemble sponginblasts and secrete the inner and outer 



horny cuticle ; others give rise to the amphidisks and 



interspersed cells of the middle layer. Under favourable 



conditions the interior cells creep out through the pore 



of the capsule, and form a spreading heap, which by 



subsequent differentiation gives rise to a young Spongilla. 



Since the freshwater sponges can only be regarded as 



modified descendants of ancient marine species (prob- 

 ably of the family Malichondridx), we may consider the 



internal gem mules, like the similar statoblasts of the 



freshwater Polyzoa, as special adaptations to a changed 



mode of life. They appear primarily to serve a protective 



purpose, ensuring the persistence of the race, since they 



only appear in extreme climates on the approach of 



drought, and in cold ones on the approach of winter. 



As a secondary function they serve for the dispersal of 



the species ; some are light enough to float down a 



stream, but not too far, so that there is no danger of 



their being carried to sea ; others, which are character- 

 ized by large air-chambers, are possibly distributed by 



the wind. 



Both sexual elements may be formed in the 



same individual, e.g., Oscarella lobularis, Grantia 



rapkanus, and many others ; but even in herm- 

 aphrodites one or other element usually occurs to 



excess in different individuals, so that some are pj^ js.— Development of a calcareous sponge (Sj/coitdra ropIranMs). o, ovum; 6,c,ovumseg- 

 predominantly male and others predominantly mented,— 6, as seen from above, c, lateral view ; d, blastosphere ; e, amphiblastula ; /, com- 

 i 1 Tii-irr J 1 1, i-cJl mencement of the invagination of the flagellated cells of the amphiblastula ; g, gastrula 



lemale. rolejaett round only one SUCn male lOrm attached by its oval face ; a, j, young sponge (Ascon stage),— A, lateral view, J, as seen from 



to 100 female forms in Grantia raphanus. In above. After Schuize. 



raphanus (tj) the nucleus of the mother -cell divides into two 

 (fig. 24 b) ; one of the resulting daughter nuclei undergoes no 

 further change, but with a small quantity of peripheral protoplasm 

 forms a " cover-cell " to the other or primitive sperm nncleus and its 

 associated protoplasm. The sperm nucleus repeatedly divides, with- 



Sexual 

 repro- 

 duction. 



Jxi above. 



other sponges — Reniera fertilis, Etcspongia officinalis — the 

 sexes are distinct. The ova develop from archseocytes or 

 wandering amoeboid cells, which increase in size and ac- 

 quire a store of reserve nourishment in the form of yolk 



