66 



TUNICATA APPENDICULARIANS 



Mertens, and by him named " Haus " ; it has recently been more 

 minutely investigated by Lohmann. It is only loosely attached 

 to the body, and is frequently thrown off soon after its formation. 

 Its function is probably protective, and possibly to some extent 

 hydrostatic, and it may also be of use in straining the nutritive 

 particles from the large volumes of water which filter through its 

 complicated passages and perforated folds.^ The long, laterally 

 compressed " tail " in the Appendiculariida is attached to the 

 ventral surface of the body (Pig. 30), and is bent downwards and 

 forwards, so that it usually points more or less anteriorly ; and is 

 twisted through an angle of 90°, so that the dorsal edge lies to 



Fig. 28. — Appendiculaiiida. A, Appendicularia sicula, Fol, with house; B, Megalo- 

 cercus abyssorurn, Chun, nat. size ; C, Oikiypleura cophocerca, Gegenb., with house ; 

 D, Fritillaria mec/achile, Fol, with vesicle ; E, Appendioularian in its house ; 

 F and G, two stages in the formation of the house. (A to D from Seeliger ; E to G 

 from Lohmann.) 



the left. It shows what have been interpreted as traces of meta- 

 meric segmentation, having its lateral muscle-bands broken up 

 into successive pieces (supposed myotomes, probably only cells), 

 while the nerve-cord presents a series of enlargements formed of 

 groups of nerve-cells from which distributory nerves are given off. 

 In Oikopleura the muscle-band in the tail is formed of ten cells 

 fused on each side. Near the base of the tail there is a distinctly 

 larger elongated ganglion. The urochord in the tail consists of 

 a homogeneous rod surrounded by a sheath containing nuclei. 



The anterior (cerebral) ganglion has connected with it an 

 otocyst (Fig. 29), a pigment spot, and a tubular richly ciliated 

 process opening into the branchial sac, and representing the dorsal 

 tubercle and associated parts of an ordinary Ascidian. The tube 

 ends in a plain or coiled cellular mass lying to the right of the 



' See Lohmann, Schrift. Naturw. Ver. Schlesw.- Hoist, xi. 1899, 347. 



