Text-book of comparative anatomy . bythe continuous division of the same ; they are the two mesodertn streaks. The larvahas now reached what is known as the Trochophora stage. It swims about freelyby means of its ciliated rings. The hindermost smaller portion of the body withthe intestine, proctodaeum, and the posterior part of the mesoderm streaks, representsthe rudiment of the afterwards segmented trunk plus the anal segment; the wholeof the larger anterior portion contains the rudiment of the head or oral segment. Larvre of the Troclwplwra type occur not only in worms, but are also commonam
Text-book of comparative anatomy . bythe continuous division of the same ; they are the two mesodertn streaks. The larvahas now reached what is known as the Trochophora stage. It swims about freelyby means of its ciliated rings. The hindermost smaller portion of the body withthe intestine, proctodaeum, and the posterior part of the mesoderm streaks, representsthe rudiment of the afterwards segmented trunk plus the anal segment; the wholeof the larger anterior portion contains the rudiment of the head or oral segment. Larvre of the Troclwplwra type occur not only in worms, but are also commonamong Molluscs. In addition to the Troclwpliora the following are some of the most importantkinds of worm larv;t-. The free-swimming larva of the Sipunculidos (Fig. 179) is already much furtherdeveloped when hatched than the Polychcctan Trocliophora, with which, however,it has much general similarity. In the Sipunculns larva the characteristicpreoral ciliated ring of the Trochophora larva is wanting, but such a ring, weakly rr md sp. .to Fio. 170.— Larva of Sipunculus, after Hatschek. o, Mouth ; sp, neural-plate ; sto, stomocteumirl-, post-oral ciliated ring; rr, retractors of the anterior body (proboscis); md, mid-gut; nuephridium ; a»i, anus. developed, is said to occur in the Phascolosoma larva. A postoral ciliated ring, onthe other hand, is strongly developed. The intestine consists, as in the Trochophnrnlarva, of stomodseum, mid-gut, and proctodseum. The latter is moved somewhatfrom the posterior end on to the back, and the postanal part of the body growingfaster than the preanal part, the anterior position of the anus which is characteristicof the adult animal comes about. The neural plate with 2, and later 4, larvaleye-spots is found in the same place as in the PoJy<:hci:tcui Trochophora. A larvalhead kidney is not formed. The mesoderm, on the contrary, is far more developedthan in the Polychcctan Trochophora. The retractors of, the anterior body (proboscis),and the
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
