534 



EVOLUTION OF ANIMALS 



Part V 



enough to warrant their status as phyla. Others are named classes by certain 

 zoologists and phyla by others. There are facts that stimulate arguments for 

 both opinions. The latter one is followed here. 



Trochophore Larvae. The trochophore is a pear-shaped larva, the stem 

 of the pear being the future posterior end of the animal (Fig. 27.1). A wheel 

 of cilia, encircles the body which also bears tufts of longer cilia, all of them 

 used in swimming. The complete U- or L-shaped digestive tube is lined with 

 cilia. The nervous system is relatively elaborate and there are various sense 

 organs such as eyes and organs of balance that might be expected on an active 

 animal. The trochophore larvae of several phyla of invertebrates have already 

 been mentioned. In annelid worms and mollusks, the trochophores are very 

 similar but the adult earthworm and clam into which they develop can hardly 

 be confused. Immature animals show likenesses; mature ones show the dif- 

 ferences. 



Phylum Nemertinea — Ribbon Worms 



Most ribbon worms live between the tide lines coiled among the rocks and 

 seaweeds; a few live in fresh water or moist earth. All of them are slender, 

 and their stretching ability is fantastic. The common Cerebratuhis lacteus of 

 muddy sands on Atlantic shores is three feet long when contracted and may 

 be 35 feet outstretched, flat and only an inch wide. Its near relative {Cerebrat- 



SIMILARITY OF YOUNG MARINE INVERTEBRATES 



A. Primitive 

 worm 



B. Annelid 

 worm 



C. Snail 



Fig. 27.1. Young stages of three marine invertebrates. A, Polygordius, a relative 

 of annelid worms. B, Echiurus, a marine worm that as an adult (4 inches long) 

 burrows in sandy bottoms. C, Patella, the limpet, a snail that clings to rocks. 

 These animals are strikingly similar in their young stages but very different in 

 habit and appearance when they are mature. As transparent, ciliated larvae they 

 swim free in the sea making their own living; in the remote past they probably 

 did so throughout their lives. {A and B after Hatschek. C after Patten. Courtesy, 

 Hesse and Doflein: Tierbau und Tierleben. Leipzig, Teubner, 1910.) 



