43 6 



THE ENTEROPNEUSTA. 



The relations of these various parts to one another are shown in a diagram- 

 matic way in Fig. 297, which represents a sagittal section of the head and thorax, 

 and in Fig. 298, which represents the entire animal viewed from the neural 

 surface. At this stage Balanoglossus may be compared to a naked phyltopod- 

 like arthropod, with the basal portion of its abdominal appendages infolded in 



the typical arachnid method to form respiratory sacs, 

 the distal portion persisting as the tongue bar. The 

 gill sacs eventually unite with, and then open into, 

 corresponding caecal outgrowths from the gut, giving 

 rise to the new type of respiratory organs characteristic 

 of the vertebrates and of several other phyla of the 

 acraniates. 



New respiratory appendages and branchial clefts 

 arise in varying numbers behind those already formed, 

 in the usual manner for segmented animals. The 

 wing-like lateral folds (Figs. 297 and 298, /./.), that 

 are so characteristic of the adults, may be compared 

 to the pleural folds on the abdominal metameres of 

 a trilobite, arachnid, or crustacean (Figs. 2, n), or with 

 the lateral folds of the trunk in a primitive vertebrate. 

 (Fig. 232.) In all these cases the pleural folds repre- 

 sent the extended lateral margins of the neural sur- 

 face, they lie lateral to the appendages and are turned 

 in a neural direction. (Fig. 297.) 



The Endocranium. We have seen that the endocra- 

 nium forms a most characteristic structure in the 

 arachnids, and in primitive Crustacea related to the 

 phyllopods. In its simplest condition, as in Apus 

 and Branchipus, it consists of a transverse bar, or 

 plate of cartilage, lying in the anterior thoracic region, 

 just back of the neurostoma, and on the haemal side 

 of the nerve cord. The main body of the endocra- 

 nium is of mesodermic orgin, and often contains two 

 kinds of cartilage-like tissues that differ in structure 

 and in their reaction to stains; one is a dense fibroid 

 tissue with small flattened nuclei irregularly distri- 

 buted; the other has more the appearance of hyaline 

 cartilage, and contains large rounded cells, in the 



peculiar grouping characteristic of certain kinds of primitive cartilage. In a 

 few cases, notably Apus, paired ectodermic ingrowths are formed, lined internally 

 with chiten, which unite with and form an integral part of the endocranium. 



These facts have an important bearing on the origin of the enteropneusta, 

 where the endocranium has a similar form and location. Here it is a flattened plate, 



FIG. 298. Diagram of Balano- 

 glossus, seen from the neural 

 surface. 



