Fig. 276.—Glossobalanus minutus, one of the higher Enteropneustans. (After Minot.)
It is perhaps not absolutely certain that the notochord of Balanoglossus and its allies is a true homologue of the notochord of the lancelet. There may be doubt even of the homologies of the gill-slits themselves. But the balance of evidence seems to throw Balanoglossus on the fish side of the dividing line which separates the lower Chordates from the worms.
It may be noticed that Hubrecht regards the proboscis of various marine Nemertine worms as a real homologue of the notochord, and other writers have traced with more or less success other apparent or possible homologies between the Chordate and the Annelid series.
Classification of Enteropneusta.—Until recently the Enteropneusta have been usually placed in a single family or even in a single genus. The recent researches of Professor J. W. Spengel of Giessen and of Professor William Emerson Ritter of the University of California, have shown clearly that the group is much larger than had been generally supposed, with numerous species in all the warm seas. In Spengel's recent paper, "Die Benennung der Enteropneusten-Gattungen," three families are recognized with nine genera and numerous species. At least seven species are now known from the Pacific Coast of North America.
Family Harrimaniidæ.—In Harrimania maculosa, lately described by Dr. Ritter from Alaska, the eggs are large, with much food yolk, and the process of development is probably, without Tornaria stage. A second species of Harrimania (H. kupferi) is now recognized from Norway and Greenland. This genus is the simplest in structure among all the Enteropneustans and may be regarded as the lowest of known Chordates, the most worm-like of back-boned animals.
Fig. 277.—Harrimania maculosa (Ritter), the lowest of chordate animals. An Enteropneustan from Alaska. (After Ritter.)
In Dolichoglossus kowalevskii the species studied by Bateson on the Virginia coast, the same simplicity of development occurs. This genus, with a third, Stereobalanus (canadensis), constitutes in Spengel's system the family of Harrimaniidæ.
Balanoglossidæ.—The family Glandicepitidæ contains the genera Glandiceps, Spengelia, and Schizocardium. In the Balanoglossidæ (Ptychoderidæ of Spengel) the eggs are very small and numerous, with little food yolk. The species in this family pass through the Tornaria stage above described, a condition strikingly like that of the larval starfish. This fact has given rise to the suggestion that the Enteropneusta have a real affinity with the Echinoderms.