GENERAL ZOOLOGY 



Contractile cells 



Fig. 9.12. Mechanisms of reception, con- 

 duction, and response in a calcareous sponge. 

 Stimulation of choanocytes sets up nerve 

 impulses which are conducted through a 

 nerve net to effectors, such as contractile 

 cells surrounding a canal. (Adapted from 

 O. Tuzet, R. Loubatieres, and M. Pavans 

 de Ceccatty, 1952, Comples rendus de I'acade- 

 mie des sciences, vol. 234.) 



Choanocytes 



vention of specialized nervous elements. Recently, students of sponges have 

 demonstrated that a primitive type of nervous system does exist, in a wide 

 variety of diflferent types of sponges. This system appears to consist basically 

 of a diflTuse network of typical neurons, with processes connecting the 

 choanocytes with contractile cells surrounding parts of the canal systems 

 (Fig. 9.12). In addition some sponges possess large and highly specialized 

 nerve cells which are lodged singly in vacuoles within the mesenchyme, and 

 others which appear from their structure and connections to function as 

 neurosensory cells. These new facts make it clear that coordination and re- 

 sponsiveness in sponges depend on the activities of cells specialized for recep- 

 tion of stimuli and conduction of impulses; presumably their functions are 

 similar to those of the more highly developed nervous systems of more ad- 

 vanced animals. The reactions of sponges are limited and sluggish but are 

 adequate for the needs of sponges, which are all either attached or incapable 

 of locomotion as adults. 



Reproduction and Development. Most sponges appear to be monoecious 

 ("one house"), or hermaphroditic, and thus are capable of producing both 

 male and female gametes. The eggs and sperms are frequently produced at 

 different times in a single individual, however, making self-fertilization im- 

 possible. In sexual reproduction the zygote develops within the parent into 

 a flagellated larva which is discharged through the osculum with the outgoing 

 water. After a brief period of free life, the larva becomes attached and under- 

 goes a peculiar type of development, in the course of which the flagellated 



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