THE COMPLEX NERVOUS SYSTEM OF ASGIDIAN LARV.E. 69 



tiny rod-shaped papillse 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. Ill 

 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, PL 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 Yertebrata, it takes its origin as an open furrow stretching 

 longitudinally along the dorsal side of the body. The edges gradually 

 grow upwards and inwards to meet, and thus form a tube which now 

 constitutes the central nervous system of the animal. 



The anterior end gradually swells into a large vesicle, while from 

 the hinder end of this, there runs a rapidly narrowing tube which is 

 prolonged into the tail as the caudal nerve. The large anterior 

 vesicle contains two peculiar sense organs, an eye, and an otolith 

 perhaps of auditory value. These can readily be made out in 

 mounted specimens, as both are darkly pigmented and show 

 conspicuously, rendering easy the location of the cerebral vesicle 

 lying between the mouth and the atrial opening. Of the two, the 

 eye is much the more complex, for it possesses a cup-shaped retina 

 in which is placed a projecting and complex lens. It is placed at 

 the posterior upper corner of the vesicle and projects downwards 

 towards the centre of the cavity. The otolith on the other hand, 

 rises upon a stalk from the floor of the vesicle more to the front end. 

 It is noteworthy that both are within the vesicle.' 



The after history of the animal is quickly told. The larva 

 affixes itself by the glutinous secretion of pne or all of its three, 

 papillae to some object ; the tail with its contained' notochord, as well 

 as the three anchoring papillae become absorbed ; the test becomes 

 enormously, thickened and strengthened, and the perforations of the 



