PHYLUM PORIFERA. SIMPLE MULTICELLULAR ANIMALS 



97 



demonstrated and their behavior is what 

 one would expect in the absence of nerves.* 

 However, they are able to respond to certain 

 stimuli; the response, as in the Protozoa, is 

 one-celled. The pores and oscula are sur- 

 rounded by contractile cells (myocytes) 

 which are able to close these openings. A 

 finger placed in an osculum may be squeezed 

 with the force of a grip of the hand by a 

 man. Apparently the myocytes respond to 

 direct stimulation, since no nervous tissue 

 is present. The entire body may contract and 

 then expand. Reactions to stimuli are very 

 slow since they depend upon the funda- 

 mental properties of protoplasm, that is, 

 conductivity and contractility. Since proto- 

 plasm can only contract and not extend it- 

 self, most movement must be due to con- 

 traction of the protoplasm; and when cells 

 elongate, it is due to the transverse contrac- 

 tion of protoplasm that decreases the width 

 of the cell and causes it to become longer. 

 But usually a return to normal by contractile 

 cells is due to simple relaxation and a con- 

 sequent return to normal shape. 



Reproduction 



Reproduction in Scypha takes place by 

 both sexual and asexual methods. In the 

 latter case, a bud arises near the point of 

 attachment, finally breaks free, and takes up 

 a separate existence. 



The sexual reproductive cells in sponges 



* While it is true that Tuzet and deCeccatty 

 have reported the presence of a primitive nervous 

 system in some sponges, this has not been con- 

 firmed at this writing by others, although several in- 

 vestigators in America are working on the problem. 

 The evidence of these two investigators for inter- 

 preting the cells in question as nerve cells comes 

 entirely from cytologic work. To the author's knowl- 

 edge there is no unequivocal staining method for 

 distinguishing nerve cells from other types of cells. 

 As their critics have pointed out, the diffuse net- 

 work of neuronlike cells demonstrated can be inter- 

 preted as connective tissue cells. Tuzet and deCec- 

 catty are now well aware that it will be necessary to 

 have parallel evidence from physiologic studies in 

 order to prove the presence of nerve cells in sponges. 

 Until such proof is available, the writer will assume 

 that the question of nervous tissue in sponges is still 

 an open one. 



lie in the jellylike layer (the mesoglea) of 

 the body wall. Both eggs and sperms occur 

 in a single individual; i.e., Scypha is monoe- 

 cious (hermaphroditic). Tlie fertilized egg 

 segments by 3 vertical divisions into a pyra- 

 midal plate of 8 cells. A horizontal division 

 now cuts off a small cell from the top of 

 each of the 8, the result being a layer of 8 

 large cells crowned by a layer of 8 small cells. 

 The cells now become arranged about a cen- 

 tral cavity, producing a blastulaiikc sphere. 

 The small cells multiply rapidly and de- 

 velop flagella, while the large cells become 

 granular. The small cells become partially 

 grown over by the others, forming a struc- 

 ture called the amphiblastula (Fig. 49); this 

 escapes from the parent as a flagellated 

 larva. After the larva swims about for several 

 days it becomes attached to a solid object 

 and begins growth as a young sponge. 



A peculiarity in the embryogeny of cer- 

 tain sponges is this: the flagellated cells of 

 the larva do not become the outer (dermal) 

 layer, as do the flagellated cells of certain 

 higher animals, but they produce the layer 

 of choanocytes; and the nonflagellated cells 

 do not become the inner (gastral) epithe- 

 lium, as do the similarly situated cells in the 

 coelenterates, but they produce the dermal 

 layer as well as the middle region. No sponge 

 has anything like an ectoderm or an endo- 

 derm as do the other metazoans. 



OTHER PORIFERA 



Form, size, and color 



Sponges may be simple, thin-walled, 

 tubular structures like Leucosolenia (Fig. 

 46), or massive and more or less irregular 

 in shape. Many sponges are indefinite masses 

 of tissue encrusting the stones, shells, sticks, 

 or plants to which they are attached; others 

 are more regular in shape and attached to 

 the sea bottom by means of masses of 

 spicules. The form exhibited by the mem- 

 bers of certain species may vary somewhat, 

 depending on whether they develop in shal- 



