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ends. The branchial sac is pierced by two oblique bands of stig- 

 mata (Fig. 23 5^). The life history is very complicated. The egg 

 develops into a tailed larva, which develops into a " nurse " ; the 

 latter is asexual, and produces three kinds of buds on a stolon, viz. 

 (1) nutritive buds which provide the " nurse " with food, (2) foster 

 forms which are set free as cask-shaped bodies with eight broad 

 muscle-bands, and (3) sexual forms which are attached for a time 

 to the foster forms, but which later become free and give rise 

 to the egg. 



Order III. — Laevacea. 



The Larvacea are very minute Tunicata which live at the surface 

 and swim by means of a tail-like appendage, resembling in this and 



Fig. 24. 



Oikopleura cophocerca in its "house" (after Fol) ; seen from right side, x 6. 

 Arrows indicate course of the water ; x, lateral reticulated parts of the " house." 



certain other respects the tadpole larva of other Tunicata. They 

 are able to form a temporary test or " house " many times larger 

 than the body (Fig. 24). The organism itself, which is almost lost 



