ill. 9 



LARVACEA 



73 



Fig. 42. Oikopleura, one of the Larvacea, in its house, showing 



the feeding-currents. 



e. exhalant aperture; e.e. 'emergency exit'; f. p. filter pipes; f.tv. filter 



window; g. gill-slit; m. mouth; r. trough; ta. tail. (After Garstang; 



this and Figs. 44 and 45 by permission of the Editors of the Quarterly 



Journal of Microscopical Science.) 



individual builds a 'house', by secretion from a special part of the skin, 

 the 'oikoplastic epithelium'. The tail is a broad structure held at an 

 angle to the rest of the body; its 

 movement produces a current in 

 which the food is carried and caught 

 by a most elaborate filter arrangement 

 ixi the house (Fig. 42). Water enters 

 the house by a pair of posterior 

 'filtering windows' and is passed 

 through a system of filter pipes in the 

 part of the house in front of the 

 mouth. The very minute flagellates 

 of the nanonplankton are stopped by 

 these pipes and sucked back to the 

 mouth. The pharynx has two gill-slits, 

 also an endostyle and peripharyngeal 

 bands. The general organization is that 

 of a typical ascidian tadpole, and there 

 can be no doubt that these forms have 

 arisen from tunicates by the accelera- 

 tion of the rate of development of the 

 alimentary organs and gonads so that 

 the metamorphosis and normal adult 

 stage are eliminated. This may, of 

 course, have happened long ago, so 

 that the modern Larvacea are not FlG - «■ Appendicular* t seen from 



the side and from below. 



closely related to any living forms, (After Lehmann.) 



