MOLLUSCA 



151 



and the posterior a continuation of the inner gill-plate. 

 There is no embryological evidence to support this sug- 

 gested connexion, and, as will appear immediately, the 

 history of the gill -plates in various forms of Lamelli- 

 branchs does not directly favour it. Yet it is very prob- 

 able that the labial tentacles and gill -plates are modi- 

 fications of a double horseshoe -shaped area of ciliated 

 filamentous processes which existed in ancestral Mollusca 

 much as in Phoronis and the Polyzoa, and is to be com- 

 pared with the continuous prae- and post-oral ciliated band 

 of the Echinid larva Pluteus and of Tornaria (49). 



The gill-plates have a structure very different from that 

 of the labial tentacles, and one which in Anodon is singu- 

 larly complicated as compared with the condition presented 

 by these organs in some other Lamellibranchs, and with 

 what must have been their original condition in the ances- 

 tors of the whole series of living Lamellibranchia. The 

 phenomenon of " concrescence " which we have already had 

 to note as showing itself so importantly in regard to the 

 free edges of the mantle-skirt and the formation of the 

 siphons, is what, above all things, has complicated the 

 structure of the Lamellibranch ctenidium. Our present 

 knowledge of the interesting series of modifications through 

 which the Lamellibranch gill-plates have developed to their 

 most complicated form is due to R. Holman Peck (50) 

 and to Mitsukuri (51). The Molluscan ctenidium is typi- 

 cally, as shown in fig. 2, a plume-like struc- 

 ture, consisting of a vascular axis, on each 

 side of which is set a row of numerous la- 

 melliform or filamentous processes. These 

 processes are hollow, and receive the venous 

 blood from, and return it again aerated into, 

 the hollow axis, in which an afferent and an 

 efferent blood-vessel may be differentiated. 

 In the genus Nucula (fig. 134), one of the 



FIG 130. Psammabiajtorida, right side, showing expanded foot e, and g incnrrent and g 1 exenrrent Arcaceae, we have an example of a Lamelli- 



branch retaining this plume-like form of gill 



the mouth w, and the two left gill-plates are reflected 

 so as to show the gill-plates of the right side (rr, rq) pro- 

 jecting behind the foot, the inner or median plate of each 

 side being united by concrescence to its fellow of the 

 opposite side along a continuous line (aa). The left inner 

 gill-plate is also snipped so as to show the subjacent orifices 

 of the left nephridium x, and of the genital gland (testis or 

 ovary) y. The foot thus exposed in Anodon is a simple 

 muscular tongue-like organ. It can be protruded between 

 the flaps of the mantle (fig. 124, (1), (2)) so as to issue 

 from the shell, and by its action the Anodon can slowly 

 crawl, or burrow in soft mud or sand. It has been sup- 

 posed that water is taken into the blood-vessels of the 

 Anodon through pores in the foot, and in spite of opposi- 

 tion this view is still maintained (Griesbach, 47). In fig. 

 124, (2) the letters ab, ac, ad, point to three pit-like depres- 

 sions, supposed by Griesbach to be pores leading into the 

 blood-system. According to Carriere (48) these pits are 

 nothing but irregularities of the surface ; in some cases 

 they are the entrances to ramified glands. Other Lamelli- 

 branchs may have a larger foot relatively than has Anodon. 

 In Area it has a sole-like surface. In Area too and many 

 others it carries a byssus-forming gland and a byssus- 

 cementing gland. In the Cockles, in Cardium, and in 

 Trigonia, it is capable of a sudden stroke, which causes 

 the animal to jump when out of the water, in the latter 



genus to a height of four feet. In Mytilus the foot is 

 reduced to little more than a tubercle carrying the aper- 

 tures of these glands. In the Oyster it is absent alto- 

 gether. 



The labial tentacles of Anodon (n, o in fig. 124, (3), (5) ) 

 are highly vascular 

 flat processes richly 

 supplied with nerves. 

 The left anterior ten- 

 tacle (seen in the 

 figure) is joined at 

 its base in front of 

 the mouth () to the 

 right anterior ten- 

 tacle, and similarly 

 the left (o) and right 

 posterior tentacles 

 are joined behind the 

 mouth. Those of 

 Area (t, k in fig. 132) 

 show this relation to 

 the mouth (a). These 

 organs are character- 

 istic of all Lamelli- 

 branchs ; they do not 

 vary except in size, 

 being sometimes 

 drawn out to 

 streamer-like dimen- 

 sions. Their appear- 

 ance and position suggest that they are in some way 

 related morphologically to the gill-plates, the anterior 

 labial tentacle being a continuation of the outer gill-plate, 



FIG. 131. Diagram of a view from the left side of 

 the animal of AnodotUa cygiwea, from which the 

 mantle-skirt, the labial tentacles, and the gill- 

 filaments have been entirely removed so as to 

 show the relations of the axis of the gill-plumes 

 or ctenidia g, A. a, centre-dorsal area ; ft, ante- 

 rior addnctor muscle; c, posterior addnctor 

 muscle ; d, month ; t, anus ; /, foot ; g, free por- 

 tion of the aids of left ctenidium; k, axis of 

 right ctenidium ; fc, portion of the axis of the 

 left etenidium which is fused with the base of 

 the foot, the two dotted lines indicating the 

 origins of the two rows of gill-filaments ; m, line 

 of origin of the anterior labial tentacle ; n, ne- 

 phridial aperture ; o, genital aperture ; r, line 

 of origin of the posterior labial tentacle. (Ori- 

 ginal.) 



In other Arcaceaa (e.g., Area and Pectunculus) the lateral 

 processes which are set on the axis of the ctenidium are not 

 lamellae, but are slightly-flattened, very long tubes or hol- 

 low filaments. These fila- 

 ments are so fine and are 

 set so closely together 

 that they appear to form 

 a continuous membrane 

 until examined with a 

 lens. The microscope 

 shows that the neighbour- 

 ing filaments are held to- 

 gether by patches of cilia, 

 called " ciliated junc- 

 tions," which interlock 

 with one another just as 

 two brushes may be made 

 to do. In fig. 133, A a 

 portion of four filaments 

 of a ctenidium of the Sea- 

 Mussel (Mytilus) is repre- 

 sented, having precisely } 

 the same structure as 

 those of Area. The fila- 

 ments of the gill (cteni- 

 dium) of Mytilus and 

 Area thus form two 

 closely set rows which 

 depend from the axis of 

 the gill like two parallel 

 plates. Further, their structure is profoundly modified by 

 the curious condition of the free ends of the depending 

 filaments. These are actually reflected at a sharp angle 



/r 



10. 132. View from the ventral (pedal) as- 

 pect of the animal of A rca A'oar, the mantle- 

 flap and gill- filaments having been cut away, 

 a, mouth ; b, anus ; c, free spirally turned 

 extremity of the gill-axis or ctenidial axis 

 of the right side ; d, do. of the left side ; 

 e, / anterior portions of these axes fused 

 by concrescence to the wall of the body ; 

 g, anterior adductor muscle ; ft, posterior 

 adductor; f, anterior labial tentacle; t, 

 posterior labial tentacle : J, base line of the 

 foot ; m, sole of the foot ; 11, callosity. 

 (Original.) 



