each of which is defined by a particular and usually more com- 

 plex pharynx structure. The highly vacuolated central paren- 

 chyma can be seen in sections to be more or less sharply distinct 

 from the denser peripheral parenchyma. The terms "en- 

 docytium" and "ectocytium" have been used for these two 

 tissues since cell membranes are hard to distinguish in them. 

 However, since some recent studies with electron microscopy 

 have shown that many turbellarian tissues, once thought to be 

 syncytial, are truly cellular, the use of these as general terms may 

 be appropriate no longer. See Ax (1961) for a long discussion of 

 this point and also Boguta and Mamkaev (1972) and Ivanov and 

 Mamkaev(1977). 



The parts of the hermaphroditic reproductive system are 

 important in identification of genera and species. In the key, an 

 attempt has been made to use only parts which can be 

 determined without sections, but this is not always possible. 



The female parts of an acoel may consist of only the develop- 

 ing and mature eggs. Thus female accessory organs may be 

 totally lacking or there may be present an organ for storing and 

 releasing sperm received from a partner (a seminal bursa or bur- 

 sa seminalis). In a few species, a vagina is present. There are no 

 oviducts in any of the acoels. The large size of the mature egg is 

 due to its being entolecithal, i.e., each egg develops and stores 

 nutritive material (yolk) in its cytoplasm. Entolecithal eggs are 

 found in some other turbellarian orders (Archoophora), but the 

 more usual condition in so-called higher Turbellaria 

 (Neoophora) is for smaller ectolecithal eggs with the nutritive 

 materials developed and stored in separate and conspicuous 

 yolk glands. Thus the acoels lack yolk glands although in a very 

 few species, Hallangia proporoides, Polychoerus caudatus, 

 Nadina pulchella, and some species of Oligochoerus, a group of 

 yolk-carrying cells near the eggs have been described. Thus, in 

 most acoels, the eggs increase in size gradually as they mature 

 and yolk accumulates so that a series of eggs of increasing sizes 

 can be seen arranged conspicuously along the length of the 

 body. The youngest stages are found in a definite area of the 

 body (a female germinal center) that is characteristic for each 

 species. This germinal center may be considered to be the ovary 

 although there is no ovary wall or delimiting capsule and the 

 center may be more or less diffuse. The term "ovary" is 



variou.sly used in the literature to indicate either the germinal 

 center or the entire mass of developing eggs. In some species, 

 there may be a common germinal center where both eggs and 

 sperm start their development in close proximity and then move 

 apart as they mature (Fig. 16). The seminal bursa may be only a 

 loosely defined vacuolated space in the parenchyma (Fig. 138) or 

 it may be a more or less elaborate structure with epithelial or 

 muscular walls (Figs. 29, 30). Nozzles (Figs. 42, 43) or spermatic 

 ducts (Figs. 62, 63) or bursal appendages (Figs. 99, 100, lOI) 

 may be present. The bursa may open directly from the female 

 pore (Figs. 69, 99), from a common genital pore (Fig. 30), or 

 from an associated female antrum or vagina (Fig. 29). Groups 

 of sperm associated with bursal nozzles may also be found in the 

 parenchyma without any discernible outside connection (Figs. 

 46, 72a, 72b) in which case it may be postulated that sperm are 

 stored in this way after they have been deposited either on the 

 outside of the body or through the epidermis by hypodermic 

 injection. However, it has been suggested also that sperm 

 deposited in a vagina or antrum may move through the wall to 

 form such a disconnected bursa or to lie in vacuolated spaces in 

 the parenchyma. 



The sperm develop in a male germinal center (testis) which, 

 like the ovary, is not delimited by a definite wall. The developing 

 stages may be arranged in a rather compact mass, may appear to 

 be organized in follicles as in some other Turbellaria, or may be 

 more or less scattered ("diffuse testis"). Distinctions between 

 these types of testes and their exact definitions are not clear (see 

 Steinbock 1966:84-85). As already noted, the male and female 

 germinal centers may be closely associated. In most cases, 

 however, the sperm develop dorsal to the eggs and move 

 posteriorly in the dorsal parenchyma, since there are no sperm 

 ducts in the male system of the acoels. 



Some sort of male copulatory complex is always present 

 although it may consist of nothing more than a space in the 

 parenchyma where sperm accumulate close to a male genital 

 pore (Fig. 74). Such an unwalled space in the parenchyma where 

 sperm accumulate before ejaculation is termed a "false seminal 

 vesicle" and such may occur not only with a simple genital pore, 

 but also with many other types of copulatory apparatus (Figs. 

 80, 169, 171). A waUed structure where sperm are accumulated 



male germinal center digestive parenchyma 



frontal gland 



seminal bursa 



sperm entering 



seminal 

 vesicle 



statocyst 

 bocJy parenchyma 



female germinal center 



mouth 



mature egg cell 



female antrum 



male antrum 



Figure 2. — Generalized diagram of acoel siructures, longitudinal reconstruction. 



3 



