330 ORDER CHOANO-FLAGELLATA. 



membranous collar and single centrally enclosed flagellum ; contractile 

 vesicles two or three in number, posteriorly located ; endoplast spheroidal, 

 subcentral. Inhabiting salt and fresh water. Increasing by transverse 

 fission, and by the breaking up of the entire body-mass into sporular 

 elements. 



This newly established generic group comprehends the simplest known represen- 

 tatives of the Choano-Flagellate order. All its members, while agreeing essentially 

 in structure with the isolated zooids of the previously discovered genera Codosiga and 

 Salpingceca, are to be distinguished from the former by their eminently solitary 

 mode of growth, and from that of Salpingaca by the entire absence of a protective 

 sheath or lorica. With relation to the compound type Codosiga, Monosiga may be 

 said to occupy a position similar to that which subsists between the solitary Peritri- 

 chous genus Vorticella and the compound forms Carchcsium and Epistylis. In all 

 the species here enumerated it has been observed that the body-sarcode is of much 

 softer and more plastic consistence than obtains in either Codosiga or most other 

 generic representatives of the same order ; owing to this circumstance it is found 

 that while each specific type preserves a more normally maintained characteristic 

 form, the separate zooids are subject to considerable individual variation. A like 

 plasticity, developed, however, to a more extensive degree, is especially distinctive 

 of the aggregated collared monads of all sponge-stocks, and which, examined in 

 their isolated condition as shown at PI. VIII. Figs. 2-7, 10, 18, and 20, might 

 easily be mistaken for members of the present genus. A distinctive feature per- 

 taining to the developmental phenomena of Monosiga as compared with Codosiga, is 

 afforded by their transverse in place of longitudinal plan of subdivision ; the anteriorly 

 produced resultant of such process of segmentation swims off as a simple collarless 

 uniflagellate monad, and forms an independent attachment. 



A. PEDICLE ABSENT, RUDIMENTARY, OR NON-PERSISTENT. 

 Monosiga angustata, S. K. PL. II. FIGS. 31 AND 32. 



Body very attenuate, clavate or subcylindrical, about four times as long 

 as broad, attached by its more slender posterior extremity, without the 

 intermedium of a pedicle; endoplast spherical, subcentral; contractile 

 vesicles two in number, posteriorly located. Length of body 1-2500". 



HAB. Fresh water, solitary. 



Only two or three examples of this elegant little animalcule have been as 

 yet observed, being then discovered attached to examples of a species of Cyclops 

 obtained from a pond on Wandsworth Common. The earlier condition of this 

 type, prior to the development of the characteristic collar, represented at PI. II. 

 Fig. 32, is remarkable for its conspicuous resemblance to the undeveloped and 

 elongate collarless monads of a motile sponge-gemmule, as illustrated in various 

 figures of PL IX. 



Monosiga consociatum, S. K. PL. IV. FJGS. 19-21. 



Body ovate or pyriform, widest posteriorly, about one and a half times 

 as long as broad, attached sessilely or through the intermedium of a short 

 rudimentary pedicle ; endoplast spherical, subcentral ; contractile vesicles 

 two in number, posteriorly located. Length of body 1-4000" to 1-3500". 



HAB. Fresh water, gregarious. 



The zooids of this species are not unlike those of the persistently stalked marine 

 M. ovata, but are of even more plastic consistence. Although a short pedicle was 



