14 NERVOUS SYSTEM AND SENSE ORGANS 
kind of smooth muscle fiber. As a result of their contraction the 
opening into the sponge is lessened or closed. 
Wilson, 1910, describes membranes covering the subdermal 
cavity and containing pores. This so-called membrane is composed 
of an external portion and is believed to be syncytial. There are 
two somewhat independent devices for the closure of pores, the pore 
membrane and the pore canal sphincter. The closure of the pore 
canals is dependent upon the sphincter-like band of cells on the 
wall of the canal. These cells are in every way comparable to a 
primitive form of smooth muscle-fiber. They are in contact with 
the water passing into the canal and seem capable of direct stimu- 
lation. The pore membrane is less muscle like and is perhaps of a 
more primitive type. 
Parker, 1910 and 1919, considers the sponges as an important 
group in illustrating the most primitive condition of the nervous 
system of metazoans. Muscle cells the independent effectors, as 
illustrated by the sphincters of sponges, were the first neuromuscu- 
lar organs to appear. The special receptors in the way of sense- 
cells were next to appear in certain coelenterates while in other 
forms more complex, the adjuster or central organ was added. 
LITERATURE 
Bidder, G. 
1896. The Collar-cells of Heterocoela. Q. Jour. Mic. Se. n.s. vol. 38, 
pp. 9-48. pl. 2. 
Lendenfeld, R. Von 
1885. Das Nervensystem der Spongien. Zool. Ang. Bd. 8, pp. 47-50. 2 fig. 
1887. Synocils, Sinnesorgane der Spongien. Zool. Anz. bd. 10, pp. 142- 
145. 2 text fig. 
Minchin, E. A. 
1900. Sponges. A Treatise on Zoology edited by E. R. Lankester. Part 
2, chap. 3. 
Parker, G. H. 
1909. The Origin of the Nervous System and its Appropriation of Ef- 
fectors. Pop. Se. Mo. vol. 75, pp. 56-64. 
1910. The Reactions of the Sponges with a Consideration of the Origin 
of the Nervous System. Jour. Exp. Zool. vol. 8, pp. 1-41. 
1919. The Elementary Nervous System. Monog. Exp. Biol. pp. 1-229. 
53 illus. 
Robertson, M. 
1911. The Division of the Collar Cells of the Calearea Heterocoela. 
Q. Jour. Mic. Sc. n.s. Vol. 57, no. 226, pp. 129-139, pl. 19. 
