12 ANATOMY AND PHYSIOLOGY 



the whole of the Spongiadse, and a great portion of them, 

 when adtdt, are so well characterised by their form as to 

 enable the student, when once well acquainted with their 

 pecuUarities, to assign each readily to its proper place in the 

 sponge. In many cases they preserve the same form from 

 the earhest to the latest period of their development, while 

 in others the variations they undergo during their growth 

 are very remarkable. It is therefore necessary that these 

 mutations of form should be carefully noted whenever they 

 are observed, lest they be mistaken for normal ones. 

 Some of the most remarkable changes in form, during the 

 course of their development, will be described under their 

 respective heads. 



The spicula in the skeletons of the Spongiadse appear to 

 be the homologues of the earthy deposits in the bony 

 structm-es of the more perfectly developed living forms. 

 In the higher tribes of animals we find the disintegrated 

 condition of the earthly deposits in the first stages of the 

 development of the bony structures in the form of minute 

 radiating patches, which in a more advanced stage unite 

 and form the sohd mass of bone, as in the mammalian 

 tribes of animals, while in the cartilaginous tribe of fishes 

 these radiating centres of bouy secretion never attain a 

 higher degree of development, but remain isolated points of 

 bony structure during the whole of the life of the animal. 

 And in the compound tunicated animals we find the cal- 

 careous stellate and sphero-granulate forms of spicula 

 developed in close accordance with the similar siliceous 

 forms in various species of sponges. Thus the stellate and 

 cylindro-stellate spicula of the sarcode in the Spongiadse 

 are apparently the homologues of the bony centres of 

 development in the higher animals. It is so likewise with 

 the other forms of sponge spicula. We find isolated 

 calcareous spicula of an irregular fusiformi-acerate shape, 

 representing the bony skeleton of the higher animals in 

 the outer integuments of several species of Boris. 



Messrs. Alder and Hancock, in their admirable ' History 

 of the British Nudibranchiate MoUusca,' describe calcareous 

 spicula occurring in Doris aspera, Ulamellata, and Triopa 



