. The Cambridge natural history. Zoology. ENDOCYCLICA 537 described by the Sarasins,^ who regarded its structure as a proof that Echinoidea were derived from Holothuroidea. Both palae- ontology and embryology have, however, yielded strong evidence that Echinoidea were derived from Asteroidea, and hence there is ground for believing that Holothuroidea are descended from primitive Echinoidea and not vice versa. The Echinothuriidae may perhaps be regarded as showing the first steps in the change, and though possibly not closely related to the actual amb. peristome. Fig. 239.—View of peris
. The Cambridge natural history. Zoology. ENDOCYCLICA 537 described by the Sarasins,^ who regarded its structure as a proof that Echinoidea were derived from Holothuroidea. Both palae- ontology and embryology have, however, yielded strong evidence that Echinoidea were derived from Asteroidea, and hence there is ground for believing that Holothuroidea are descended from primitive Echinoidea and not vice versa. The Echinothuriidae may perhaps be regarded as showing the first steps in the change, and though possibly not closely related to the actual amb. peristome. Fig. 239.—View of peristome of Asthenosoma hystrix. ami, Ambulacral plates on the lower edge of the corona ; inter, lower plates of the interambiilacral area. (From Wyville Thomson.) ancestors of the Holothuroidea, they at any rate show parallel modifications. Fam. 3. Saleniidae.—Endocyclica with a large peristome and periproct. The peristome is covered with thin, scattered, irregular plates. There are five pairs of special buccal tube-feet, each supported by a special plate, and there are external gills. The periproct is excavated in the side of a large central pentagonal plate. It is covered with fifteen or twenty plates.^ The ambulacral plates are separate as in the Cidaridae, but occasionally adhere in pairs near the peristome. The inter ambulacral plates also, as in Cidaridae, each bear one large primary spine surrounded by a ' Reference on p. 528 n. 2 account of the periproct is different from that ordinarily given. It is based on the most recent examination of this family-Agassiz, " Panamic Deep- ^ea Echini," Mem. Mus. Gomp. Zool. xxxi. 1904, p. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Harmer, S. F. (Sidney Frederic), Sir, 1862- ed; Shipley, A. E. (Arthur Everett), Sir, 1861-1927. ed. [London
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