ASCID1ACEA DEVELOPMENT OK THE FREE-SU I M MING LARVA. 363 



somewhat loosened ; they assume a spherical form and constitute a 

 mesenchyne which tills the primary body-cavity of the trunk-section 

 (Fig. 168, ms). 



We have seen (p. 350) that, according to van Beneden and Julin, 

 the mesoderm in this anterior part heroines detached from the ai-ch- 

 enteron in the form of paired coelomic 

 diverticula (Figs. L 59 and ISO A). The 

 true eoelom that arises in this way 

 belongs, however, merely to the earliest 

 embryonic stages, and disappears even 

 as early as the time when the mesoderm 

 completely separates from the entoderm. 

 The mesoderm then tills the space 

 between the ectoderm and the entoderm, 

 which, in later stages, becomes consider- 

 ably widened and, as it appears, filled 

 with a gelatinous mass ; this forms the 

 -round - substance of the mesenchyme 

 that arises through the transformation 

 of this anterior portion of the mesoderm. 

 The lacunae which arise later in this 

 mesenchyme must be regarded, like 

 the blood-vessels (which, according to 

 van Beneden and Julin are entirely 

 devoid of an endothelial wall) as the 

 pseudocoele. 



The mesenchyme of the trunk-region, 

 which is derived in the above wav, yields 

 the mesodermal organs of the adult 

 AAcidian. Its histological differentiation 

 uives rise to the connective tissue, as 

 well as to the pigmented elements, 

 and to the body-musculature of the 

 adult which appears arranged in radial 

 and circular muscles surrounding the 



inhalent and exhalent orifices as well as into longitudinal muscles 

 <>f the trunk, etc. Single cells of the mesenchyme, which become 

 tree and reach the pseudocoele, become changed into blood- 

 corpuscles. We shall see later (p. 380) that the genital organs 

 also, with their efferent ducts, and the urinary organs originate in 

 the mesoderm. 



FlG. 169. — Transverse section 

 through the caudal portion oi 

 the free - swimming larva of 

 ( Vavelina (after Seeliger). ch, 

 chorda ; ee, ectoderm : Jt, 

 median tin ; m, cellulose 

 mantle ; mf, muscle-fibrillae in 

 transverse section : »>".. muscle- 

 cells; />/•. neural tube; s, sub- 

 chordal entoderm - strand ; . 

 mantle- cells. 



