Primary Study On The Early Development In Urechis Uniconctus

Urechis uniconctus, which subjects to Echiuroidea, Echiura, Xenopeusta, Urechidae, is a kind of economic marine invertebrate. Because of its arguable classification and high sulfide tolerating ability, it gets more and more attention. In this paper, the early development of Urechis uniconctus was studied using histological, histochemical, SEM, TEM, biochemical and isozymic methods to do some primary work for its better knowledge.After artificial fertilization, the first and second polar body was released from the egg. The blastula was formed by spiral cleavage. The gastrula, which was formed by invagination, was hatched at 24h pf (post-fertilization). The trochophore had two parts of evident hemispheres, including the typical apical cilia tuft, prototroch and telotroch. At about 30~35d pf, the posterior hemisphere of larva began to elongate and segment. Here the larva stepped into the somite larva. After the settlement and metamorphosis, the somite larva lost its somite, and developed into the worm-shaped larva. At 2 month pf, the juvenile formed. The whole early development lasted for about 2 month.The ontogenesis and differentiation of digestive canal and body wall in Urechis uniconctus was also studied using histological, histochemical and TEM methods in the laboratory. The results showed that, the digestive canal initiated from the gastrocoele. When the digestive canal opened at early trochophore, the larva began to feed. Here the digestive canal could be divided into esophagus, stomach, midgut and hindgut according to the canal diameter. The body wall was formed only by unilayer cells. After the settlement, the whole canal developed and differentiated evidently. Until the worm-shaped larva, the tela submucosa and tunica muscularis came forth atesophagus, stomach and hind gut. The craw and gizzard could be defined. The muscle fibre could be found in the midgut. The aspiratory intestine could be distinguished from the midgut by the nucleus/cytoplasm ratio. As the worm-shaped larva grew, the muscle layer, cuticle and mucus cells formed orderly. When the larva developed into juvenile, the pharynx differentiated at the anterior part of esophagus, and the whole digestive canal was extremely similar with the adult in the structure and function, consisting of pharynx, esophagus, craw, gizzard, stomach, midgut, aspiratory intestine and hindgut. And now, the structure of body wall was almost same with the adult, consisting of the epithelium, muscle layer and coelomic membrane. Furthermore, in the histochemical study, the different parts of digestive canal and body wall showed different activity of nonspecific esterase, alkaline phosphatase, acid phosphatase, ATPase, lipase and mucopolysaccharide.Furthermore, the isozymic and biochemical characteristics of Urechis uniconctus larvae before metamorphosis were studied using the technique of polyacrylamide gel vertical plate eletrophoresis and the method of biochemistry. The isozymic results showed that, all 5 isozymes became complicated with the larval development. EST was the most complicated, increased from 1 band at gastrula stage to 6 bands at somite larva stage. MDH could be detected at all the experimental stages. At the stage of gatrula, there showed no activity in AMY, LDH and ALP. The biochemical study of ACP, ALP, PO, POD and CarE indicated that, the content of all these 5 enzymes grew higher during early development. All the above results indicated that, the feeding behavior played an important role in stimulating the enzymic activity quantitatively and qualitatively. At the mid-trochophore, the digestive ability in lipid became higher than before, and the immunocompetence of late trochophore and somite larva was obviously enhanced.