MICROSCOPICAL STUDIES. 105 



In size, the colonial individuals are usually very much smaller 

 than the simple, but in structure the two are essentially similar, 

 having the anterior portion of the alimentary canal distended into a 

 large chamber, with walls perforated by numerous gill slits, a nervous 

 system reduced to a single rounded mass (ganglion) between mouth 

 and cloacal aperture, a blood system of much simplicity and an 

 enveloping case of nearly structureless semi-cartilaginous or gela- 

 tinous tissue, the tunic or test. 



July and August is the breeding season of many species, and if 

 one of these animals be then dissected, eggs and embryos in various 

 stages of development will be found. 



Taking one of the most advanced, the following points can be 

 observed. The body is a tiny oval, slightly flattened from side to 

 side, while from the one end is given off a long broad tail, containing 

 centrally a clear rod of a cartilaginous substance. Both the body and 

 the tail are invested in a thick coat of gelatinous material, equivalent 

 to the test of the adult fixed animal. Turning the animal on to one 

 side (when the tail is found to be now on edge) one can, by staining, 

 make out clearly the thick investing test, in which lies the darker stained 

 kernel-like body. At five points the test is broken through ; three 

 of these are where, at the anterior end, there pass an equal number of 

 tiny rod-shaped papillae spreading into wide heads on their emer- 

 gence. These are glandular in function, secreting a tough glutinous 

 cement whereby the larva when tired of a free life, attaches itself 

 to a rock or weed. The other two points where the test is pierced, 

 are upon the dorsal side, and in the larva of Aplidium elegans are 

 situated far back. The anterior is the opening of the mouth (o), 

 and is separated by only a short interval from the posterior, which 

 is a depression indicating where the atrial aperture breaks 

 through. The mouth opens into a wide pharynx occupying a great 

 part of the larval body space. In the stage figured, the stomach 

 is just beginning to be apparent as a thick walled canal in a ventral 

 position towards the hinder part of the body, and connection is 

 just being made between the atrial cavity and the intestine. In 

 the advanced larva several openings pierce the pharynx and 

 allow water entering through the mouth to pass into the atrial 

 cavity without having to traverse the stomach and intestine. Such 

 openings are obviously the same as are so numerous and so apparent 

 in the adult. In Fig. 4, PI. x, they show as two rows of disc-shaped 

 thin places in the pharyngeal walls. 



The nervous system, of the most extreme simplicity in the adult, 

 has in the larva a complicated and highly developed arrangement. 

 As in the Vertebrata, it takes its origin as an open furrow stretching 



