54 THE MOST PRIMITIVE METAZOA 



There are certain exceptions to the generalisation that sessile 

 animals are radially symmetrical. Thus, as previously mentioned, 

 even amongst the coelenterates there are many planes that will not 

 divide the animal into two equal halves, this being due to the 

 development of tentacles, gonads, batteries of nematocysts, 

 mesenteries and siphonoglyphs. In other Metazoa it is rare to find 

 radial symmetry. Thus in the Rotifera, neither Trochosphaera 

 nor Melicerta are perfectly radially symmetrical. In the Annelida 

 Sabella is not radially symmetrical; its parapodia still show a 

 bilateral symmetry. In the barnacles, though there is some 

 tendency towards a radial symmetry as illustrated by the skeletal 

 plates, the internal symmetry of other organs such as the legs, 

 digestive system and nervous system is a bilateral one. Other 

 sessile animals such as the Crinoids, Ascidians and Pterobranchi- 

 ates do not show perfect radial symmetry. On the other hand the 

 ctenophores that take up a crawling habit such as Coeloplana and 

 Ctenoplana do show a very interesting bilateral (biradial) symmetry. 



These examples indicate that subject to certain basic limita- 

 tions, the life that an animal leads will influence its shape and 

 basic symmetry. The question, " Is radial symmetry more 

 primitive than bilateral symmetry? " should perhaps be more 

 correctly replaced by the question, " Is the sessile or pelagic habit 

 more primitive than the swimming and crawling habit? " The 

 answer to the latter question is at present unknown. 



(1) The Sponges (Porifera) 



The sponges are peculiar multicellular animals with an organisa- 

 tion quite different from that of the other Metazoa. They have a 

 skeletal system and three layers of cells, pinacocytes, amoebocytes 

 and choanocytes, but they have no organ systems such as an 

 excretory or a nervous system. They have a very simple digestive 

 system in which there is no real mouth or gut. The sea water 

 around the animal passes through a series of apertures into the 

 centre of the sponge and in doing so is filtered, the food being 

 taken up by the choanocytes and the amoebocytes. 



The organisation of the sponge is very simple in that a sponge 

 can be passed through the meshei of a net and so separated into its 

 individual cells. These cells can later aggregate and form an 

 organised sponge with the cells in their correct relative position; 



