GLOCHIDIA^ — FACTORS UNDERLYING ENCYSTMENT 467 



Just posterior to the adductor muscle, on either side of the 

 median plane, is a symmetrical grouping of cells from which the 

 gut, foot, nephridia, and other organs will develop during meta- 

 morphosis (fig. 3). 



3. Axe-head glochidia. There are four species of mussels, 

 historically placed in the genus Lampsilis on the basis of super- 

 ficial adult shell characters, which possess peculiarly shaped 

 glochidia. These constitute the axe-head type — so-called from 

 their suggestive resemblance in surface view (figs. 5 and 6). 

 The four species producing it have been known as Lampsilis 

 alata, capax, laevissima, and purpurata, but Simpson ('00) and 

 particularly Sterki ('95, '03), Ortmann ('11), and Utterbach 

 ('16) have elevated them, on the basis of larval characteristics 

 and adult internal anatomy, to a distinct genus, Proptera. Here 

 they doubtless belong; at any rate the glochidia deserve a sepa- 

 rate grouping. Except for P. laevissima (Coker and Surber, 

 '11) larvae of the axe-head type possess hooks, which, however, 

 are said by Lefevre and Curtis ('12) not to be homologous with 

 those of Anodonta and its allies, but are to be regarded as more 

 nearly related to the hookless forms. 



B. Natural history. When ripe, the glochidia are free from the 

 egg membranes, and, in most species, are not united with any 

 firmness by the mucus or jelly which binds them into compact 

 'conglutinates' at earlier stages of development. Indeed, the 

 accepted criterion of 'ripeness' and suitability for artificial infec- 

 tion purposes is based on the degree of freedom of the individual 

 larvae. On the contrary, the glochidia of Strophitus are 

 expelled embedded in gelatinous cords and those of Obliquaria 

 in cylindrical masses. 



According to Lefevre and Curtis ('10, '12), the ripe glochidia, 

 usually held loosely in slimy strings by a mucus secreted from 

 the marsupial epithelium, are discharged through the exhalent 

 siphon at irregular intervals. They sink to the bottom and come 

 to rest with their widely gaping valves open upward. The 

 mucus, when present, is dissolved in a short time, leaving the 

 larvae as entirely separate units. 



