THE RISE OF THE LARVA, ETC. 129 



phore, viz., the paired primitive (larval or head-) kidney. This organ, 

 as already noted, is conspicuous in the larval Lamellibranchs and in 

 the Annelida (c/. p. 39 and Vol. i., p. 267). It has not, indeed, been 

 discovered in Patella, but we may reasonably expect that it will be 

 found in this form which in most other points is so primitive, 

 especially as it is found in other Gastropods of a less simple type of 

 development, such as the fresh-water Prosobranchia (Bythinia, Pahi- 

 diita, p. 136) and the Pulmonata (p. 178). A tubular primitive 

 kidney has recently been described as occurring in the larva of an 

 unidentified marine Gastropod (v. Erlanger, No. 28). The 

 primitive kidneys in their original form appear as tubular structures, 

 the relation of which to the primary body-cavity is probably the 

 same as in the Lamellibranchs, and these organs open outwards on 

 the ventral side of the body behind the velum. These primitive 

 excretory tubes are either quite short (Paludina, Fig. 59 B, un, p. 139) 

 or else longer, as in Planorbis, in which case each kidney consists 

 of a Y-shaped tube (Fig. 78, un, p. 177). 



Besides the primitive tubular kidney, various groups of ectoderm-cells have 

 been claimed as primitive excretory oi-gans. Bobretzky thus interpreted two 

 rounded cell-growths which appear near the rudiment of the foot. Similar 

 organs have been found by McMurrich in Fulgur (No. 70). Sarasin de- 

 scribes, in Bythinia, ectoderm-cells of excretory nature which are connected 

 with the velum. [In Crepidula, Conklin (No. IV.) finds paired groups of ecto- 

 dermal cells, situated just behind the velum, which are eventually cast off ; he 

 regards them as excretory.] These ectodermal cells frequently contain concre- 

 tions which are said to be extruded, a fact which has led authors to attribute 

 an excretory function to them. They are very soon to be recognised owing to 

 their granular contents ; in Neritina, such granular cells, which later give rise 

 to velar cells, may be clearly distinguished even during cleavage among the mic- 

 romeres (Blochmann, No. 7). Two rows of granular cells which lie along the edge 

 of the velum have been described in Onchidium by Joyeux-Laffuie (No. 51).* 



*[Heymons describes, in Umbrella, the presence of paired groups of ecto- 

 dermal excretory cells, situated near the anus ; of these, the right group alone 

 attains functional development and sinks under the surface of the ectoderm. 

 He regards these as homologous with the similar cells situated near the velum 

 in the Prosobranchia. Conklin (No. IV.), however, thinks that they are 

 only analogous, since they arise from totally distinct blastomeres ; the anterior 

 ectodermal excretory cells are found in three of the great Gastropodan orders. 

 Mazzarelli (No. XV.), who has studied the ectodermal or anal kidney of 

 Aplysia, considers that it is not to be regarded as a larval organ; he main- 

 tains that it does not disappear, but represents the rudiment of the definitive 

 kidney. The true internal primitive kidney is so far known to occur only in 

 the Pulmonata and in two fresh-water Prosobranchia and possibly in one 

 marine Gastropod, concerning which Erlanger is unable to inform us whether 

 it was an Opisthobranch or a Prosobranch. Thus it will be seen that this 

 supposed primitive organ is found to be most highly developed in those most 

 specialised forms, the Pulmonata, and that it is only elsewhere known to 

 occur in two Prosobranchs. A further search for this organ in some of the 

 more primitive marine Prosobranchia is much needed. — Ed.] 



K 



