102 BALANOGLOSSIDA 



and possibly in the Vertebrata become segmented into the 

 mesoblastic somites. 



Finally, we must not forget to call attention to the great 

 variability presented by different members of the group. This 

 variability not only occurs among different species, but is also 

 shown by different individuals of the same species. Moreover, 

 and this is the remarkable point, it affects features of structure 

 which to judge by the standard of higher Chordata are of great 

 importance, and are usually perfectly constant through large 

 groups. In illustration of this we may call attention to the 

 condition of the central canal and dorsal roots in the nerve cord 

 of the collar (many species of Ptychoderidse), to the canals and 

 pores which connect the proboscis coelom with the exterior 

 (species of Ptychodera), and to the length of the branchial region 

 as compared with the rest of the body (macro- and brachy- 

 branchiate varieties of Ptychodera) : these features are variable 

 in some cases within the limits of the same species. As examples 

 of characters which vary in allied species and which we should 

 otherwise judge to be important, we may refer to the presence 

 or absence of liver diverticula, to the condition of the notochord, 

 and to the presence or absence of external protective covering of 

 the gill-slits, and finally to the presence or absence of genital 

 pleurae. It is in consequence of this remarkable variability that 

 in our treatment of the group we have entered into much greater 

 detail than has been our custom in this work. 



Farn. 1. Ptychoderidae. Proboscis usually shorter than the collar: 

 cornua of proboscis skeleton do not extend backwards beyond the middle 

 of the collar. Dorsal unpaired roots unite the medullary cord of the collar 

 with the epidermis. Efferent vessels of proboscis united in one transverse 

 plane by a circular vessel with the ventral blood vessel of the collar. Peri- 

 pharyngeal spaces contain circular muscles and completely surround the 

 buccal cavity continuously up to the level of the mouth opening. Peri- 

 haemal cavities without transverse muscles. Circular muscles outside the 

 longitudinal are usually present in the body wall of the trunk. Hypo- 

 pharyngeal streak as a well-marked groove on the ventral side of the 

 pharynx (Fig. (il). Genital pleurae well developed or small. Lateral 

 mesenteries present in the trunk coelom. External liver-saccules present 

 (except in (llnssobalanus ruficollis, Willey). Ptychodera Esch., the gill- 

 slits open directly to exterior and the genital pleurae have a ventral 

 origin, generally with continuous axial canal in the medullary cord of the 

 collar, Pt. fiava Esch., erythraea Speng., bahamensis Speng. Balanoglossus 

 D. Chi., gill-slits open into pouches which discharge to exterior by dorsal 

 i_nll pm-es, genital pleura* with dorsal origin, B. apertus Speng., clavigerus 

 D. Chi., gifja* F. v. Mull., aurantiacus Girard, australiensis Hill, carnosus 



