4 PHORONIDEA. 



ectoderm and entoderm. At the most posterior ])art of the streak 

 lies the small pit mentioned above (Fig. 1 D, g ; Fig. 2 C, g) ; this 

 dej^ression soon gives rise to two lateral diverticula that extend 

 anteriorly between the ectoderm and entoderm (Fig. 2 C, m'), and 

 l)ecome the posterior coelomic sacs. The two pairs of coelomic sacs 

 (m' and m") that thus arise are connected together by the isolated 

 mesoderm -cells which have arisen from the anterior ])art of the 

 primitive streak. After the formation of the posterior pair of 

 coelomic sacs, which may, perhaps, be in some way connected with 

 the formation of the nephridia, a shallow ectodermal depression 

 forms posteriorly and fuses with the wall of the archenteron, and 

 here the anal aperture arises. 



According to Metschnikoff (No. 9) and Foettinger (No. 5), the mesoderm' 

 forms much earlier tlian is stated by Caldwell, by a kind of mesenchyme- 

 formation, single cells appearing in the cleavage-cavity of the blastula-stage 

 (Fig. 1 A, B, a-). These elements, according to the authors just named, are 

 rather small, so that Caldwell's assumption that particles of protoplasm 

 lying iu the blastocoele have here been mistaken for mesoderm-cells appears 

 .somewhat probable. It is, however, possible that the formation of the coelomic 

 s.ics is ])receded by the rise of a mesenchyme. 



According to Roule also (No. 9a), single mesenchyme-cells are found as 

 early as the gastrula-stage in the primary body-cavity. Later, after the anal 

 aperture has formed, the cells of the primary entoderm are said to increase 

 ill number at the .sides of the anus, and in this way to produce two solid 

 mesoderm-bands, while single cells that become detached mingle with the rest 

 of the mesenchyme. 



The anterior pair of coelomic sacs above described, which might 

 be called cephalic cavities, now grow out anteriorly, and soon com- 

 ])letely fill the interior of the pre-oral lobe. These sacs seem to 

 yield only the lophophoral cavities (Fig. 5, Ih) and the connected (?) 

 cavity in the epistome (Fig. 5, eh) of the adult, which are separated 

 by a transverse septum from the posterior part of the body-cavity.* 

 This latter is yielded by the posterior pair of coelomic sacs, from 

 which also is derived the median mesentery suspending the intestine 

 (Fig. 5, ms) which is retained throughout life. Secondary, lateral 

 mesenteries, however, are also found. 



With the development of the pre-oral lobe, the principal sections 

 of the alimentary canal and the coelomic sacs, the chief parts of 

 the embryo are represented in rudiment, and now the whole surface 

 becomes clothed with cilia. Thickening of the ectoderm now takes 



* [According to Mastermax (No. II.), there are three perfectly distinct 

 coelomic cavities in the larva, viz. — a pre-oral or epistomal cavity, a collar- or 

 lophophoral-cavity, and a trunk-cavity. Their origin is not described. — Ed.] 



