go6 THE ANIMAL KINGDOM. 



in the Upper Chalk ; the Tetraxonina are sparingly represented in Carbo- 

 niferous strata and the Chalk. Shells, bored, as it is supposed, by a Clione, 

 occur in Jurassic, Cretaceous and Tertiary formations, and the spicules of 

 a Spongilla (Sp. Purbeckensis) have been detected in freshwater limestones 

 of the Purbeck series (Jurassic). 



The mesoglaea is soft or firm, hyaline or granular ; it contains cells of various 

 kinds, and sometimes fibres. The cells are either naked or provided with a delicate 

 membrane. They fall under the following heads : (i) fusiform or stellate cells, 

 the latter sometimes connected by their processes ; (2) pigment cells ; (3) vacuo- 

 lated cells ; (4) amoeboid cells of variable shape and wandering habit, sometimes 

 becoming granular and then vacuolate, or converted into hyaline, refractile, lumpy 

 bodies of irregular shape with fat-like contents (Chondrosia), or partially filled by a 

 starch-vacuole 1 . Fibres or fibrils united in bundles or lamellae, and apparently 

 "non-nucleated, occur in some instances, e. g. in Chondrosia, but never in Calcarea. 

 Other forms of cells are the spongoblasts, the spicule-bearing cells, gland-cells, con- 

 tractile fibre cells and nervous cells. The gland-cells are saccular or pear-shaped, 

 disposed in a single layer below the ectoderm, towards which one or more granular 

 processes extend. They are found in many Non-Calcarea and some Leucone Cal- 

 carea, and have been most completely investigated in Aplysilla violacea and 

 Dendrilla by von Lendenfeld (Z. W. Z. xxxviii. pp. 254-6; 278-80). He found 

 that if the ectoderm were injured, the cells in question secreted a slimy substance 

 (? spongin), hardening under water in about twenty-four hours into a cuticle, 

 beneath which a new ectoderm and layer of gland cells were developed. The con- 

 tractile fibre cells are fusiform, nucleated, and in Euspongia canaliculata (=anfrac- 

 tuosa] showing traces of transverse striae, more or less regular 2 . They are found 

 sparingly in Calcarea Heterocoela, but are very common in the Non-Calcarea, round 

 the pores and oscula, accompanying the inhalent canals or the bundles of spicules, 

 e. g. in Tethya, or in the cortex (p. 807), or surrounding the fibres of the skeleton in 

 Aplysilla violacea and Dendrilla. Nervous elements have only been detected 

 recently : their character and position leave little doubt as to their real nature. 

 There are two forms of them, the palpocil and synocil. The former is a delicate 



1 The granules in the granular cells appear to be albuminoid ; cf. Keller, Z. W. Z. xxx. 

 pp. 570-2 ; fat-like bodies are described in Chondrosia by Schulze, Z. W. Z. xxix. p. 104; starch- 

 containing or amylum cells in various sponges by Keller, op. cit. pp. 572-6. Brandt considers that 

 the starch is due to the presence of algae : see Mitth. Zool. Stat. Naples, iv. pp. 232, 296. For 

 starch vacuoles and granules in Spongilla, see Ray Lankester, Q. J. M. xxii. p. 241. Carter states 

 that starch is met with in the ova of marine sponges as well as in the gemmules of Spongilla 

 (A. N. H. (5), xii. 1883). Krukenberg has found an ethereal oil in Chondrosia and some other 

 sponges (Vergl. Physiol. Studien, (i), ii. p. 42 et seqq.). 



a These cells resemble in form the smooth or non-striped muscle-cells of many Metazoa. They 

 are certainly endued with contractility, and the closure, whether complete or incomplete of the pores, 

 &c., or movements of the surface of a sponge, are, beyond doubt, due to their action. They are called 

 contractile fibre cells because they have no connection with a nervous system ; but, if recent dis- 

 coveries are correct, and there is connection between the sense, ganglion, and fibre-cells of the pores 

 or canal system, as von Lendenfeld supposes in some instances, then they may be termed muscle- 

 cells. It is difficult to say what their function may be when they surround the skeletal fibres as in 

 Dendrilla, 



