SOUTHERN OYSTER DRILL AS A PREDATOR 



79 



buflFers (Lillie, 1948, p. 260-263), depenclinfr„n the 

 fixative used, the procedure produced brilliant 

 differential staining of the two types of digestive 

 gland follicular cells, necrotic areas in the host 

 tissues, and various parasite-stnictures. In a few 

 instances, Weigert's acid-iron-chloride-hematoxy- 

 lin was substituted in the procedure, but the re- 

 sults were less satisfactory. 



THE PARASITE 



Taxonomy 



The adult was originally named Zeugorchis 

 acanthus by Nicoll (1906), who described it from 

 two specimens fomid in the cloaca and bursa 

 Fabricii of the herring gull (Lanis argentatus) . 

 Following study of more material from herring 

 and common gulls {L. canus), he (1907) desig- 

 nated it type of a new genus, Parorchu. Subse- 

 quently, Linton (1914) mistakenly described the 

 same worm as a new species, P. avitus. Lebour 

 (1907) described Cercarln pvrpurae from Purpura 

 { = Thais) lapUluJi and subsequently (1914), on 

 morphological grounds alone, correctly identified 

 it as a larval stage of P. acanthxis. Later, Stmik- 

 ard and Shaw (1931) described Cercaria senmfera 

 from UrosaJpinx cinerea and Stunkard and Cable 

 (1932) demonstrated experimentally that C. xensi- 

 fera is a larval stage of P. avitus. Cable and 

 Martin (1935) reduced P. avitm Linton, 1914, to 

 synonymy with P. acanthus (Nicoll, 1906) Nicoll. 

 1907. To the synonym list compiled by Cooley 

 (1957) should be added the genus Proctobhim 

 Travassos, 1918 (cited in Strom, 1927). 



The systematic position of P. acanthv.9 is illus- 

 trated by the following classification scheme taken 

 from Hyman (1951) : 



Phylum Platyhelminthes 

 Class Trematoda 

 Order Digenea 



Family Echlnostomatidae 

 Genus Par»rchis Nicoll, 1907 

 Parorchis acanthuii (Nicoll, 1906) Nicoll, 1907. 



Morphology 



The morphology of the different developmental 

 stages of P. acanthus has been described by a num- 

 ber of authors. A complete source-list of the de- 

 scriptions was given in an earlier paper (Cooley, 

 1957). The material of the present study is in 



general agreement with these descriptions. The 

 existing differences, however, do not invalidate 

 the identification of the present material, for, as 

 Stunkard (1957, p. 16) pointed out, ''membere of 

 a single species may difl'er so much as a result of 

 development in different host species, invertebrate 

 and vertebrate, or of different physiological condi- 

 tions in host-individuals, that the extent of varia- 

 tion is known for few if any species. . . ." 



Miracidium. — The present material is compared 

 with published descriptions in table 1. The main 

 differences are (1) the variation in length of an- 

 terior, body, and caudal cilia, (2) the variation in 

 the shape of pigment spots ("eyespots," "eyes") 

 [No two descriptions agree on this], and (3) the 

 shorter length of the contained redia. In addi- 

 tion, Nicoll (1907) reported the body to be dif- 

 ferentiated into a distinct head and a posterior 

 part, but neither Linton (1914), Rees (1940), nor 

 the author have observed such a condition. It was 

 obviously a temporary shape, perhaps as a result 

 of contraction during fixation. The miracidium 

 swims rapidly, yawing slightly as it rotates about 

 its longitudinal axis. "V^Hiile swimming, both 

 miracidium and contained redia become elongate, 

 but regain their typical shapes upon halting. In 

 other respects, there is agreement with published 

 descriptions. 



Table 1. — Comparison of living P. acanthus miracidia 

 front several sources 



[All measurements in millimeters) 



