26 ILLINOIS BIOLOGICAL MONOGRAPHS [220 



have been added. This is the most important genus of the family, since it 

 is by far the largest both in number of species and number of individuals. 

 Altogether, there are sixteen species, one being a salt-water form. Then, 

 too, the feeding habits make it seem to command an economic importance. 



The most noticeable details of structure are the rather large sensory pits 

 on either side of the head. These are unique and extraordinary in themselves 

 and probably take the place of the statocyst, eyes, and sensory organs of other 

 types. The blunt, somewhat stiff anterior end and the protruding mouth 

 region give a special shape and appearance to the head. The comparatively 

 large intestinal cavity, filling so nearly the integumental sac is an other con- 

 spicuous character. Altogether, these forms are worthy of m.ore than ordinary 

 interest. 



Stenostoma leucops (Ant. Duges) 1828 

 Figures 50 and 51 



Of the sixteen species v/hich make up the genus, the oldest, Stenostoma leu- 

 cops (a. Duges) 1828 has had the most attention. The original description 

 was full and exact. Almost nothing more was done with it until H. N. Ott 

 in 1892 made a careful and complete study of the histology of all the body sys- 

 tems, bringing out a number of details of value for comparative work with 

 other families. Since this form is most primitive and evidently nearest the 

 ancestral type, all characters of structure are of especial interest. In an ex- 

 perimental way, several authors have made extensive researches. C. M. 

 Child during 1901-2 and 1903 published a series of five papers dealing with the 

 development of the zooids and regulation of fission using both this species and 

 Stenostoma grande. His results explain, at least to some degree, some of the 

 life-habits and variations always to be noticed. Since the asexual reproduction 

 is the general method, individuals in all stages of regeneration and development 

 are to be found together. One or two of his general conclusions throw light 

 on the laws which govern these common processes. Perhaps the most im- 

 portant are the following: When a zooid separates from the chain thru injury, 

 "complete destruction of younger parts by older, may occur, but not older 

 parts by younger." Again, ''if the separation from the parent is very early, 

 the anterior portion of the younger individual develops a brain regardless 

 of its former relationships." These two facts will account for many of the half 

 changed shapes and conditions present in smaller specimens. Ritter and 

 Congdon (1900) have also used this convenient form as the basis of a series 

 of experiments having to do with fission induced artificially or inhibited in 

 some ma,nner. These authors emphasized the migration of the brain ganglion 

 and also certain special laws of regeneration which seem to hold. Child does 

 not agree with these findings and brings forth rather exhaustive data to prove 

 the truth of his own conclusions. 



The use to which this species has been put is due partly to the sim.plicity 

 of structure, and consequent simplicity of reaction, partly to convenient 



