106 



COMPARATIVE ANATOMY. 



Fig. 37. 1 Cell with 

 .1 siliceous spicule of 

 Spougilla. 2 Ves- 

 icle with an amphi- 

 disc of Spougilla 

 (after N. Lieber- 

 kiihn) . 



(Fig. 37, 2). The siliceous spicules are ofteu greatly elongated, 

 and form excessively delicate skeletons (Euplectella), or they form 



bulky structures which project as tufts of fila- 

 ments far beyond the body (Hyalonema). Lastly, 

 in the Fibrospongia?, the skeleton of the body 

 is formed by fibres united into a network, which 

 consist of a substance allied to chitin. 



In the Acalephas also the deposition of in- 

 organic substances in the mesoderm leads to the 

 formation of various kinds of skeletons. In the 

 Anthozoa they generally have the form of colonies, 

 and the hard substance is almost always formed 

 of calcareous salts. These give rise to deposits 

 (Fig. 45) of definite form (Fig. 38), which are 

 scattered in the soft parts of the body; or to 

 connected masses, which vary in form according 

 to their mode of development. The calcareous 

 bodies (spicula) always lie in the connective-tissue 

 of the parenchyma, and are very varied in form. They have an 

 organic basis, which retains the form of the spicula after the lime 



is removed. The connected skeletal 

 parts are formed either by the union 

 of spicula, which are connected to- 

 gether by a hard organic substance, 

 as in Corallium, or by the direct cal- 

 cification of a secreted horny sub- 

 stance, which lies in the axes of the 

 ccenenchyma, and does not possess 

 spicula. When the organic sub- 

 stances predominate the skeleton is horny, as it is in the Gor- 

 gonidre and Antipathidrc. These axial skeletons are sometimes 

 limited to the trunk of the colony, as in the Pennatulida3, where 

 they lie in the shaft of the stock, or they may be continued into 

 all the branches of the stock. There is another form of skeleton, 

 which resembles the axial ; it is formed by the gradual calcification 

 of the parenchyma of the body, in which process spicula sometimes 

 take part. In this way the aboral portion of the whole body is 

 more or less completely hardened. A proportionate forward growth 

 of the body at its oral pole occurs at the same time, and the parts 

 which are completely calcified represent the dead base. Skeletons 

 of this kind form the calcareous supports of the Fungia3, Astra3ida3, 

 Madrepores, and of Tubipora. The skeleton thus formed must be 

 regarded as a continuation and development of the skeletons which 

 are found in the Spongiae. 



Fig. 38. 



Calcareous Spicules of 

 Alcyonium. 



82. 



Another kind of supporting organ is formed in the interior of 

 the body by cuticular structures, or by differentiation of more resistent 

 connective substances. The simplest example is again found in the 



