26 NATURAL ARRANGEMENT OF INSECTS. 



and which form the immediate connection between such 

 vertebrate animals as Amphioocus and Myxine among 

 fish, and such annulose animals as Porocephalus^ and 

 other white-blooded Fermes as have the sexes distinct. 

 He divides the whole into five principal groups : . 

 1. The Nereidina, or such as are free, having a distinct 

 head, provided either with eyes, or antennae, or with 

 both. 2. Serpulina, the animals of which are se- 

 dentary, destitute of a head, but provided with eyes or 

 antennae. These two are considered the typical groups, 

 and are distinguished by having their body provided 

 with distinct feet. 3. The Lumbricina are animals 

 without eyes or antennae, having the body setigerous 

 for locomotion, and the articulations distinct. 4. The 

 Nemertina, which are aquatic animals, without eyes or 

 antennae, the body not externally setigerous, and the 

 articulations indistinct. 5. and lastly, the Hirudina 

 are animals provided generally with eyes, but not with 

 antennae : the body is not externally setigerous, but the 

 articulations are distinct. To these definitions Mr. 

 MacLeay has added the following observations on three 

 of these groups, which are too valuable to be omitted. 

 The Nereidina he considers as the most perfect in their 

 structure of all the class, as they possess numerous 

 organs, and have a distinct head, which is generally 

 provided with eyes and antennae : some of them, after 

 the manner of the Serpulina, inhabit tubes, which tubes 

 are membranaceous, and formed by a transudation from 

 their body ; but in general the Nereidina are naked, 

 and they are always agile animals, freely moving about 

 in search of their prey. In general appearance they 

 are wonderfully like centipedes. 2. The Serpulina of 

 our author are sedentary animals, without eyes or an- 

 tennae : they live in tubes, which are either a natural 

 transudation of their body, of a membranaceous or cal- 

 careous substance, or their tubes are semifactitious, or, in 

 other words, composed of an agglutination of particles of 

 sand, or other small substances. The calcareous nature 

 of the tube in some Serpulina is very advantageous for 



