1884.] 105 [Hyatt. 



derm in the true cloaca is an intermediate step towards the for- 

 mation of a stomodeum, and from it we may possibly get a hint 

 of the mode in which the stomodeum and proctodeum might 

 have arisen from the simplest form of an enlarged intercellular 

 pore. The pore having arisen by pressure from within, and as an 

 outlet naturally enlarging beyond the size of the inlets, would 

 form the primitive cloacal osculum of the gastro-vascular arch- 

 enteron. When the mesoderm thickened and the outlets multi- 

 plied, the lip of the original outlet might become the limit of the 

 true endoderm, and this would agree with the limits of the endo- 

 derm in Marshall's larva, and the limits observed in adult Ascones 

 or Sycones. In other forms, however, purely mechanical out- 

 growths of the periphery of the upper surface would take place, 

 and the vase form of cloaca arise, or when carried to excess the 

 true tubular cloaca would appear also lined by the ectoderm. The 

 continued inheritance of this ingrowth of the ectoderm, whether 

 taking place in one way or another, and the inheutance of the ten- 

 dency to form ingrowths which must surely follow in descend ent 

 species, would tend to produce just such organs as the invaginations 

 which give rise to the stomodeum. This accounts for the origin of 

 the stomodeum in the higher forms, as a blind pit in a suitable 

 locality near the archenteron, though apparently at first entirely 

 independent, and for its elongation in the proper direction and 

 final opening into this cavity. As an ectoblastic invagination 

 having these peculiarities it becomes impossible to account for 

 them in the embryo unless the stomodeal pit was previously con- 

 nected with the archenteron, or formed the outer part of a tube 

 leading into that cavity. Otherwise we cannot understand its 

 choice of a suitable location and its internal growth in an evi- 

 dently selected and predetermined direction towards the archen- 

 teron. 



The tubes which connect the ampullae or coelomic cavities 

 with the exterior are true water tubes and are doubtless the 

 homologues of the tubes which connect the water vascular or 

 coelomic cavities in Echinodermata with the exterior. These 

 last are, however, outgrowths of the coelomic sacs and may be 

 considered as the homologues of the primitive tubes of the Porif- 

 era only so far as these are formed by the endoderm. These 

 opinions seem to harmonize in the main quite closely with 



