CHAP. X. CLASSIFICATION. 109 



CLASSIS CRUSTACEA. 



Sub-class I. HOLOSCHISTA. Segmentation complete. No primitive 



band. Nauplius-brood. 

 Ord. 1. Ceratometopa. Nauplius with frontal horns. (Cirripedia, 



Rhizocephala.) 

 Ord. 2. LEIOMETOFA. Nauplius without frontal horns. (Cope- 



poda, without Achtheus, &c., Phyllopoda, Peneus.) 



Sub-class II. HEMISCHISTA. Segmentation not complete. 



A. Nototropa. Embryo bent upwards. 



Ord. 3. Protura. The tail is first formed. (Mysis.) 

 Ord. 3. Saccomorpha. A maggot-like larva-skin is first formed. 

 (Isopoda.} 



B. Gasterotropa. Embryo bent ventrally. 



Ord. 5. Zoeogona. Full number of limbs not produced in the egg. 



Zoea-brood. (The majority of the Podophthalmata.) 

 Ord. 6. Ametabola. Full number of limbs produced in the egg. 



(Astacus, Gecarcinus, Amphipoda less Hyperia?) 



This sample may suffice. The farther we go into 

 details in this direction, the more brilliantly, as may 

 easily be imagined, does the naturalness of such an 

 arrangement as this force itself upon us. 



All things considered, we may apply the judgment 

 which Agassiz pronounced upon Darwin's theory, with 

 far greater justice to the propositions just examined : 

 " No theory," says he, " however plausible it may be, 

 can be admitted in science, unless it is supported by 

 facts." 



