216 ZOOLOGY SECT. 



gastrula larval stages constitutes an important connecting link. 

 But a closer examination of the -subject shows that some of these 

 apparent points of resemblance are superficial only, and establishes 

 a number of differences between Sponges and Ccelenterates too 

 important to allow us to suppose that a close relationship exists. 

 One of these differences stands out beyond the others as the most 

 radical. The osculum of a sponge is found, when we trace the 

 development of the larva, to correspond in no sense with the 

 mouth of the ccelenterate. The latter corresponds with the blas- 

 topore or gastrula mouth. In the Porifera the gastrula mouth is 

 (Fig. 82, p. 114) found in all cases in which the details have 

 been made out with certainty to become applied to the substratum 

 when the larva fixes itself, and the osculum is developed at the 

 opposite extremity of the body. This alone, apart from important 

 differences in the adult structure, such as the presence in the wall 

 of the sponge of the system of inhalant apertures, the presence of 

 the peculiar collared endoderm cells, and the absence of stinging 

 capsules would suffice to remove the Sponges from the Ccelenterata, 

 and place them in a phylum apart. On the other hand, that the 

 Sponges and Ccelenterates were originally derived by a common 

 root from the Protozoa i.e. possessed a common metazoan ancestor 

 is rendered very probable when we consider the similarity that 

 "xists between the members of the two groups in the earlier stages 

 of their development. 



APPENDIX TO THE CCELENTERATA 



THE MESOZOA. 



Under the designation MESOZOA have been comprised certain lowly organised 

 animal forms, formerly supposed to afford us something of the nature of a 

 connecting link between the Protozoa and the Metazoa, but now more generally 

 looked upon as degenerate members of the latter sub-division. It has been 

 proposed to term them the Planuloidea, from the resemblance which they bear 

 to the Planula larva of the Ccelenterates. 



They are all multicellular, with an ectoderm composed of a single layer of 

 cells ciliated in whole or in part, and an endoderm either composed of a single 

 elongated cell or of sevei-al cells ; a mesogkea is not represented. The Mesozoa 

 comprise three families, the Dicyemidd', the If<t< rofi/t miilic, and the Orthonectida . 

 all the members of which are internal parasites. 



The Difj/tniidn' are parasites in the kidneys of various Cuttle-fishes and 

 Octopi (Cephalopoda). The animal (Fig. 162), the length of which is between 

 0'7o and (j or 7 millimetres, consists of a head-part and an elongated body. 

 The form of the head varies a good deal, according to age : in young specimens 

 it is isotropic (i.e. symmetrical around the long axis) ; in the adult condition 

 ventral and dorsal sides are distinguishable. It consists of a swollen disc of 

 four cells and a ring of four or five pole cells. The cells of the head all bear 

 cilia, which are shorter and thicker than those of the body cells. 



The body consists of a single large axial endoderm cell and of a single layer 

 of ectoderm cells, which completely invest the axial cell. The ectoderm cells 

 which follow immediately on the head are distinguishable from the rest by their 

 granular contents, and by their being dilated internally in such a way that the 

 apex of the axial cell is constricted. Originally all the ectoderm cells are 



