THE CLOSING OF THE OLD MOUTH. 25 1 



line. Here, therefore, the adult condition of the jaws is intermediate between the 

 typical arthropod and the typical vertebrate condition, and is similar to the condi- 

 tion of the jaws in the higher vertebrate embryos. 



5. That the gill pouches of vertebrates may be interpreted as invaginated 

 respiratory appendages, which have become secondarily united with enteric 

 pouches. 



6. That the free portions of the cephalo-thoracic appendages of arthropods 

 are represented in vertebrates by embryonic oral tentacles, such as the "balancers" 

 and the external gills of amphibian larvae, and the oar-like appendages of the 

 ostracoderms. 



7. That the paired appendages of typical vertebrates, i.e., pectoral and 

 pelvic fins, arise from a new generation of post-branchial metameres that are not 

 represented in arthropods. The lateral fold from which they arise may be re- 

 garded either as a marginal fringe of rudimentary appendages or as a series of 

 keel-like pleurites. 



I. The Closing of the Old Mouth. 



It will be recalled that the alimentary canal of arthropods is formed in three 

 separate sections, the midgut arising from the endoderm, while the foregut and 

 hindgut arise from separate infoldings of the ectoderm. (Fig. 43.) 



The infolding for the foregut, or stomodseum, is always formed in the median 

 portion of the procephalic lobes (Fig. 25); the lateral cords lying on either side 

 of it, and cross commissures in front and behind. (Fig. 46, A.) As these impor- 

 tant parts of the nervous system are formed at a very early period and are never 

 known to be absent, the stomodaeum is securely trapped in a nerve ring from which 

 there is no escape. 



There are several factors in the evolution of arthropods that steadily work 

 toward the closing up of this old passageway, or which make the access to it more 

 and more difficult. 



In the more primitive arthropods there is ample room for the stomodaeum and 

 for the passage through it of a liberal supply of food. But during the phylogeny 

 of the phyllopod-arachnid stock there is a great increase in the volume and com- 

 pactness of the anterior cranial neuromeres, which narrows in a very marked 

 degree the opening between the nerve cords for the passage of the stomodseum. 

 Moreover, in all arthropods there is a tendency for the rostrum, which represents a 

 fused pair of appendages lying in front of the mouth, to gradually work its way 

 backward, thus covering up the original site of the mouth, or carrying the entrance 

 to it a long way back of its original position. (Figs. 3, 46.) It may then be 

 surrounded by projecting appendages, or it may lie at the bottom of a long atrial 

 chamber, access to which can be obtained only in an indirect or roundabout man- 

 ner, as in cirripeds (Figs. 274, 275), cladocera and phyllopods (Figs. 7, 9 and 273.) 



In many cases, the animal can never bring its mouth in direct contact with 

 its prey. Liquid foods must be pumped through long capillary tubes, formed by 



