REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 1 49 



heart, and pericardium. A backward curve in the posterior outHne at either side of 

 this mass appears to represent imperfectly developed lateral pits, from the outer borders 

 of which Schierholz (1888), Schmidt (1885), and Harms (1909) agree that the first 

 rudiments of the gills originate, and which are very conspicuous structures in the 

 glochidia of the hooked type. We have never observed any structure resembling the 

 larval thread or its rudiment in the fully formed glochidia of species of the genera 

 Lampsilis and Quadrula, the glochidia of which we have studied most extensively; and 

 the larval thread is not present in functional condition in any of the species we have 

 studied from the genera listed on page 145, with the exception of Anodonla and Unio. 

 A discussion of this organ, which has heretofore been assumed to occur in all glochidia, 

 is given after the account of the hooked glochidium which follows. 



THE HOOKED TYPE. 



Our first infections were performed with the hooked glochidium of Anodonta 

 cataracta, which is essentially like the Anodonta type of glochidium described for Euro- 

 pean species, and which has been described in a detailed manner by Lillie (1895). Our 

 later work has been with the young of Symphynoia complanaia and S. costata, the glochidia 

 of which resemble one another in structure, as shown by their outlines in text figure 1, 

 A and B, and figures 9 and 10, plate viii; so that here, as elsewhere noted in the case 

 of bookless glochidia, the outline appears to be a characteristic of the genus, which 

 enables one at once to distinguish the glochidia of Anodonta from those of Symphynoia. 

 There is, however, a marked size difference between the glochidia of these two species 

 oi Symphynoia (text fig. i, A and is). 



In both Anodonta and Symphynoia glochidia, the slightly greater length of one 

 border of the valve between hook and hinge is indicative of the future anterior region. 

 In most bookless glochidia there is a similar slight difference in the anterior and posterior 

 marginal outlines (fig. 13, pi. viii), but it is more difficult to detect, and in any case 

 the safest guide is the larval adductor muscle, which is always recognizably nearer the 

 anterior end, a position to be correlated with the location of the rudiments of the adult 

 organs in the posterior region. In the living glochidium of S. complanata the shell 

 shows calcification beneath the cuticle and is marked as though the calcareous layer 

 were porous. 



The external appearance of these hooked glochidia is like that shown for 5. costata 

 in figure 10, plate viii. The hooks, with their spines, the fibers of the lar\'al adductor, 

 and the sensory cells are seen when turned in profile view (fig. 9, pi. viii); but the 

 cellular structure is so obscure in living specimens that the rudiments appear only as a 

 denser area and even the fibers of the adductor muscle are not very distinct. There is 

 no sign of a larval thread or a thread gland, nor do sections of preserved glochidia reveal 

 such a structure. A conspicuous feature of the whole mass of glochidia in Symphynota, 

 as taken from the gill of the parent, is the thick, ropy mucus in which they are embedded. 

 This holds them so firmly together that when stirred up in a dish they remain suspended 

 and quite evenly distributed through the water, settling to the bottom only very slowly 



