282 J. HOOD ON THREE NEW EOTIFERS. 



When I jBrst saw this small Eotifer I was quite at a loss to 

 know where to place it, and several correspondents to wdiom I 

 sent specimens could not at first decide to what genus it should 

 properly belong. When swimming the creature's motions are 

 not unlike those of a Synch ^ta, minus the foot and side auricles. 

 It frequently changes its form by muscular contractions of its 

 very flexible skin, and then suggests a Sacculus or small 

 Asplanch.na, but then the head and trophi were quite different, 

 and, in addition, it carries its eggs behind, which no Asplanchna 

 or Sacculus does. I also sent some living specimens to Mr. C. 

 Eousselet, who, by the form of the head and trophi, and by its 

 internal organization, recognized it as belonging to the genus 

 Polj-arthra, although deprived of lateral appendages. IS'ow the 

 chief character of Polyarthra jplatyptera, so far the only species 

 of this genus, excepting a variety, is its cluster of skipping 

 spines on each side of the body, and it seems absurd at first 

 sight to place beside it an animal wholly deprived of this its 

 principal peculiarity. However, on closer examination, the 

 shape of the head with the two fleshy sense organs, the structure 

 of the jaws, and its whole internal organization is found to be 

 identical with that of PolyartJira platypfera, so that no choice is 

 left in the matter, and the specific name of "aptera" appro- 

 priately expresses its chief characteristic. 



The normal shape of the body is an elongated parallel-sided 

 and flattened sac, squarely truncated in front, but more rounded 

 and creased behind ; but this shape is frequently altered by the 

 contractions of a broad band of muscular threads encircling 

 the body just below the head, which can contract the body into 

 a more or less dumb-bell shape, as represented in Fig. 2a. The 

 skin also is so thin and flexible posteriorly that it is frequently 

 thrown into folds of various shapes. In P. platyptera the skin 

 is firmer and the shape more constant. 



Two pairs of flat muscles are attached to the head and 

 posteriorly to the skin, but the prominent striated muscles of 

 P. platyptera, which actuate the blades, are here quite absent. 

 The large spherical and nearly black eye is seated on the small 

 brain. The trophi are forcipate like those of P. platyptera and 

 are shown in Fig. 2b. from Mr. Rousselet's sketch. The stomach 

 and intestine, as well as the lateral canals and tags, are normal. 

 The ovary is large and generally bulges out ventrally. The 

 eggs are earned attached to the posterior, both female and 



